Hemophilia

Hemophilia is a group of inherited bleeding disorders in which blood clots form slowly or incompletely because of a deficiency in a specific clotting protein. The two main forms are hemophilia A, caused by low levels of factor VIII, and hemophilia B, caused by low levels of factor IX. Both are X-linked recessive conditions, so they mainly affect males, while a smaller number of cases arise from a new mutation or an acquired autoimmune process. Severity depends on how much functional factor remains in the plasma, and untreated severe disease causes spontaneous bleeding into joints and muscles and can be fatal when bleeding occurs in the brain.

The condition matters for three reasons. It is one of the best-characterised genetic diseases, and its study shaped the understanding of the coagulation cascade, sex-linked inheritance, and the molecular basis of protein deficiency. It also drove major advances in medical practice, from plasma-derived concentrates and recombinant factor products to non-factor antibodies and gene therapy. Finally, it raises persistent problems of access, cost, and the development of antibodies against replacement factors, which continue to shape clinical care and health-system planning worldwide.

This entry follows the disorder from its biological basis to its clinical course, diagnosis, and treatment. It covers the normal clotting mechanism, the genetics and inheritance pattern, clinical features and complications, the immune complications of treatment, and the historical discovery of the disease. It then reviews the changing therapeutic landscape, from factor replacement and prophylaxis to emerging gene-based approaches, and closes with care organisation, global access, and unresolved research questions.

Definition and classification

Hemophilia is a group of inherited bleeding disorders in which one specific clotting protein is missing or does not work properly. Factor VIII and factor IX, when faulty or absent, produce the two most common forms, which are called hemophilia A and hemophilia B. When a clotting factor falls below normal levels, clot formation takes longer, so bleeding lasts longer than it otherwise would When one of these factors is not present in normal levels, blood clotting is delayed, which results in prolonged bleeding. The defect lies in plasma proteins rather than in platelets or vessel walls, which is why platelet disorders and vascular disorders are separate categories in the differential diagnosis. That list also includes deficiencies of several other clotting factors Less Common: Factor 1 (Fibrinogen Deficiency), Factor II (Prothrombin Deficiency or Dysfunction), Factor V Deficiency, Factor VII Deficiency, Factor X Deficiency, Factor XI Deficiency, Factor XIII Deficiency, Platelet Disorders. The condition most often confused with hemophilia is von Willebrand disease, which arises from a problem with von Willebrand factor rather than with a specific clotting factor Unlike hemophilia, it is caused by a deficiency or dysfunction of von Willebrand factor rather than a specific coagulation factor.

Types of hemophilia

Hemophilia A, caused by factor VIII deficiency, is the more frequent of the two main forms. Hemophilia B, caused by factor IX deficiency, is about four times less common Factor VIII deficiency, or hemophilia A, is the most common type of hemophilia. Factor IX deficiency, or hemophilia B, is about four times less common. Hemophilia B is also known historically as Christmas disease Also called Christmas disease, hemophilia B is due to a mutation in factor IX.

Hemophilia C, caused by factor XI deficiency, is far rarer. One estimate puts its incidence at about 1 in 100,000 people in the general population The estimated incidence is 1 in 100,000 in the general population. Its bleeding typically appears after trauma or surgery and varies considerably in intensity Inherited factor XI (FXI) deficiency, also called Hemophilia C, is an uncommon autosomal recessive disorder, which is associated with a variable bleeding tendency that usually manifests after trauma or surgery.

The types also differ in how they are inherited. Hemophilia A and B are X-linked recessive, so they mainly affect males while females can carry the trait These two types are inherited in an X-linked recessive pattern, meaning they primarily affect males, although females can be carriers. Hemophilia C follows an autosomal pattern, so it affects both sexes equally Due to hemophilia C's genetic inheritance pattern, the disorder affects men and women equally, unlike other forms of hemophilia, which primarily affect men. Although it is largely recessive, heterozygous individuals can also show increased bleeding Unlike Hemophilia A and B, Hemophilia C is not completely recessive and heterozygous individuals also show increased bleeding times. Some sources also describe autosomal dominant cases in some cases hemophilia C can be caused in an autosomal dominant manner when only one copy of the mutated gene is inherited from either parent.

Severity grading

Severity is graded by residual factor activity, expressed as a percentage of normal. The reference value is taken from a measurement made before any treatment, because it reflects the person's underlying activity Baseline clotting factor activity is used to determine hemophilia severity. It is measured prior to receiving any treatment and is used because it best represents a person's inherent clotting factor activity. Normal activity generally falls within a range of roughly 50 to 150 percent The average normal factor level is 100%, with the normal range generally defined as 50% to 150%. Severe disease corresponds to activity below 1 percent, moderate disease to 1–5 percent, and mild disease to above 5 percent but below 40 percent Severe hemophilia - FVIII level less than 1% of normal; Moderate hemophilia - FVIII level 1-5% of normal; Mild hemophilia - FVIII level more than 5% but less than 40% of normal. The lower boundary of mild disease is written as 6 percent in some sources Moderate or mild hemophilia cases have FVIII levels of 0.02 to 0.05 IU/mL (2% to 5%) or 0.06 to 0.40 IU/mL (6% to 40%), respectively.

These cut-offs correlate imperfectly with clinical bleeding. Some people with activity below 1 percent have little or no spontaneous bleeding, while some with 1–5 percent present with clinically severe disease patients with FVIII levels of less than 1% occasionally have little or no spontaneous bleeding and appear to have clinically moderate or mild hemophilia. Furthermore, the reverse is true for patients with procoagulant activities of 1-5%, who may present with clinically severe disease.

Congenital and acquired forms

The primary classification axis separates congenital from acquired forms. Congenital hemophilia is an inherited deficiency of a clotting factor, caused by a defect in the relevant gene. The distinction is not absolute, however. A significant share of cases arises from a new mutation with no family history, or from an acquired immune process In a significant number of cases, the disorder results from a new mutation or an acquired immunologic process. Congenital hemophilia complicated by antibodies against the infused factor is also described as partly congenital and partly acquired Haemophilia A with "inhibitors" (that is, allo-antibodies against factor VIII) and haemophilia B with "inhibitors" (that is, allo-antibodies against factor IX) are non-limiting examples of coagulopathies that are partly congenital and partly acquired. Acquired hemophilia, which is caused by autoantibodies against the body's own factor in people without earlier bleeding, is described in a separate section.

Normal blood coagulation and the role of clotting factors

A cascade of enzyme activations

Coagulation is a chain of proteolytic reactions in which an inactive precursor, or zymogen, is cleaved into an active enzyme that acts on the next precursor in line. The chain ends with thrombin, which converts fibrinogen into fibrin; fibrin strands then assemble into the mesh that reinforces the platelet plug. Thrombin also feeds back on the system by activating factors V, VIII and XI, so the reaction accelerates once it starts. The process begins in one of two ways. The extrinsic route starts when tissue factor on cells outside the vessel meets blood after injury and binds factor VIIa. The intrinsic route begins with factor XII and proceeds through factor XI to factor IX. Both routes converge at factor X, which is the point where the intrinsic and extrinsic pathways meet. Reactions run mainly on negatively charged phospholipid surfaces, such as the exposed membranes of activated platelets, and calcium ions are required. Natural inhibitors, including antithrombin and the protein C system, limit the cascade so that clots stay localised.

Factor VIII as a cofactor and von Willebrand factor as its carrier

Factor VIII is a large glycoprotein of about 2,332 amino acids that circulates at very low concentration, mostly bound to von Willebrand factor (VWF). The VWF binding site, located in the D′D3 region of VWF, is what keeps factor VIII from being cleared rapidly, so VWF effectively extends its circulating half-life. When thrombin cleaves factor VIII at specific arginine residues, the B domain is released, VWF dissociates, and phospholipid-binding sites are exposed. The resulting factor VIIIa is not an enzyme itself. It acts as a cofactor that binds factor IXa on the phospholipid surface and holds the two in position for factor X activation. This assembly, often called the tenase complex, greatly increases the catalytic efficiency of factor IXa; published estimates of the enhancement range from roughly $10^{5}$ to $2 \times 10^{5}$-fold. The cofactor activity is short-lived, because excess thrombin, activated protein C and antithrombin inactivate factor VIIIa. VWF has a separate role in primary hemostasis, where it helps platelets adhere to exposed subendothelial collagen.

Factor IX within the intrinsic and extrinsic routes

Factor IX is a vitamin K-dependent serine protease of about 415 amino acids, synthesised in the liver and circulating as an inactive single-chain zymogen. Activation requires the removal of a short activation peptide by cleavage at two sites. This can be done either by the factor VIIa–tissue factor–calcium complex or by activated factor XI in the presence of calcium. In physiological haemostasis, the tissue factor route acts early in fibrin formation, while factor XIa-mediated activation is thought to help maintain clot integrity over time. Once formed, factor IXa is the enzyme that, with factor VIIIa, phospholipid and calcium, converts factor X to factor Xa. Factor XI, in turn, links the two routes by cleaving factor IX, so a defect in factor IX interrupts the amplification step that the intrinsic route provides.

Why the defect produces delayed, prolonged bleeding

In hemophilia, the initial phase of hemostasis is largely preserved. Platelets adhere and aggregate, and tissue factor triggers a small early burst of thrombin. The defect appears later, when the cascade needs to amplify. Without functional factor VIIIa or factor IXa, the tenase complex cannot form efficiently, so factor X activation and thrombin generation remain low. Fibrin deposition is therefore insufficient to reinforce the platelet plug, and the clot that does form is fragile. Bleeding often resumes or continues hours after an injury, when the initial clot is disturbed by movement, pressure or the body's own fibrinolysis. This explains why bleeding in hemophilia tends to be deep, recurrent and prolonged, rather than the immediate mucosal or skin bleeding typical of platelet and vascular disorders.

Genetic basis and X-linked inheritance

Location and structure of the F8 and F9 genes

Hemophilia A is caused by variants in the F8 gene, which lies on the long arm of the X chromosome in band Xq28 X chromosome. The gene is unusually large, spanning about 186 kb of the X chromosome, and is composed of 26 exons and 25 introns. Its coding region is much smaller: a 9-kb coding region contains the 26 exons, so most of the gene's length is non-coding intron sequence. The translated precursor has 2,351 amino acids, and the mature protein contains 2332 amino acids. Genome databases do not agree on exon counts; the NCBI record lists 27 exons in its total, so exon numbering should be checked against the reference transcript used in a given study.

Intron 22 is structurally unusual because it contains other genes. The first, F8A, is transcribed in a direction opposite to that of the factor VIII gene, while the second, F8B, is transcribed in the same direction as the factor VIII gene. This arrangement matters for the most common structural mutation in the gene (see below).

Hemophilia B is caused by variants in the F9 gene, which is located on Xq27 X chromosome rather than Xq28. Hemophilia A results from pathogenic variants in the F8 gene located on Xq28, while hemophilia B arises from variants in the F9 gene located on Xq27.

X-linked recessive transmission

Because both genes are on the X chromosome, both disorders follow an X-linked recessive pattern. A male has one X and one Y chromosome, so a single altered copy of the F8 or F9 gene on the X chromosome is sufficient to cause disease. A female has two X chromosomes, and often retains adequate clotting factor production from the unaffected gene copy. A female with one altered copy is termed a carrier. Carriers usually do not display symptoms because they have another normal copy of the gene, although some do have bleeding symptoms that tend to be milder.

The pattern of transmission follows directly from this. If a mother is a carrier, each son has a 50% chance of inheriting hemophilia while each daughter has a 50% chance of being a carrier. If a father has hemophilia, none of his sons inherit the mutation because sons receive their father's Y chromosome rather than his X chromosome. Instead, all daughters inherit the altered X chromosome and become carriers, and a substantial minority of these daughters have some degree of symptoms.

Mutation types and their clinical links

Hemophilia A is genetically heterogeneous. Reported counts of distinct pathogenic variants differ between sources: one review cites more than 2,000 different pathogenic F8 variants, while another reports more than 3000 different mutations in the gene. The variants include gene rearrangements, point mutations, large deletions and insertions, with rearrangements among the most frequent in severe disease mutation.

Two intronic inversions dominate severe hemophilia A. The inversion of intron 22 accounts for about 45% of severe haemophilia A and is caused by an intra-chromosomal recombination within the gene. That recombination occurs between inverted repeats found in intronic sequences in upstream regions located telomeric to the FVIII gene. The inversion originates almost exclusively in male meiosis. The intron 1 inversion is less common, found in 2%-5% of patients with severe HA. Both inversions prevent the formation of full-length FVIII messenger RNA, which explains their association with severe disease.

Other mutation types show a wider range of effects. Point mutations can lead to mild, moderate, or severe deficiency of factor VIII, based on the effect of that mutation on factor VIII gene function. Mild, moderate and severe categories are defined by residual activity, as described in the classification section genetics. Genotype also predicts inhibitor risk: certain mutation types, particularly large deletions and Inv22, are strongly associated with a higher risk of inhibitor development, and in one South Indian cohort Inv22-positive patients were found to have a significantly higher risk of developing inhibitors than those who are Inv22-negative.

Spontaneous (de novo) mutations

A substantial share of cases have no family history. About one-third of people who are diagnosed with hemophilia have no family history of the bleeding disorder. In these cases a new alteration (mutation or change) occurs in the factor VIII or IX gene. The new variant can arise in a parent's reproductive cell or early in fetal development. A study of families with sporadic non-inversion variants traced origins to the zygote within the first few cell divisions and to single sperm cells, depending on the case. Some variants are mosaic, and individuals with mosaic variants require further confirmation of the origin in the parent.

For inheritance counselling, the practical point is that a negative family history does not exclude the diagnosis. Families whose child has a de novo variant face different risk estimates from families with a known carrier, and this makes genetic counseling tricky, as the risk factors are not as clear as in cases with a known family history. Accurate counselling therefore depends on identifying the causal variant in the affected individual and, where possible, testing the parents for the same variant genetic testing.

Clinical manifestations and severity

Bleeding patterns by severity

Residual factor activity determines both the frequency of bleeding and the circumstances in which it first appears. In severe disease, defined as less than 1% of normal activity, bleeding is frequent and often occurs without an identifiable cause. Joint and muscle bleeds predominate, and internal bleeding is more likely than in milder forms. Moderate disease (1–5% of normal activity) typically produces bleeding after trauma, dental work or surgery, with spontaneous episodes occurring only occasionally. In mild disease, the usual threshold is a major injury, surgery or tooth extraction, and the first episode may not occur until adulthood. Reported cut-offs vary between sources: some place the upper limit of mild disease at about 40%, others at about 49%, so the boundaries should be read as approximate.

The timing of bleeding is also characteristic. Injury does not make a person with hemophilia bleed faster than others, but bleeding takes longer to stop, and it may resume days after an injury or operation. Deep bleeds can appear well after a minor injury, which is why the delay between trauma and symptoms is itself a diagnostic clue. Spontaneous bleeding increases in frequency with age in severe disease: in one reported series, joint bleeds made up about a fifth of hemorrhages in children aged 1 to 6 but around 60% in adults aged 18 to 65.

Hemarthrosis and chronic synovitis

Intra-articular bleeding, or hemarthrosis, is the most common manifestation of severe hemophilia, accounting for roughly 80% of musculoskeletal bleeds. The knees, elbows and ankles are most often affected. The synovium of these joints is richly vascularised, which makes it prone to bleeding, and weight-bearing stress can provoke an episode. Blood left in the joint breaks down, releasing iron that deposits as hemosiderin. This iron, together with cytokines and enzymes, drives synovial inflammation, and the thickened, hyperaemic synovium bleeds more readily after minor microtrauma. The result is a cycle of bleeding, synovitis and rebleeding.

A joint that repeatedly bleeds in the same place is called a "target joint", generally defined as one with three or more spontaneous bleeds within six consecutive months. Between episodes, reduced mobility, effusion, synovial thickening and contractures of muscle, ligament and joint capsule may be detectable. Repeated bleeding leads to progressive hemophilic arthropathy, in which cartilage is lost, subchondral bone changes, and the joint eventually becomes deformed and functionally impaired. A single acute bleed usually resolves without lasting change; the structural damage accumulates from recurrence, which is why prevention of repeated joint bleeds is central to management.

Deep muscle and soft-tissue hematomas

Bruising is common, but the characteristic problem in hemophilia is deep rather than superficial bleeding. Unlike platelet disorders, which typically cause skin and mucosal bleeding, hemophilia causes hemorrhage into muscles and large synovial joints. Large muscle groups are most often involved, including the quadriceps, iliopsoas, gluteal muscles, calves and forearms. Muscle bleeds cause pain, swelling and loss of movement, and an expanding hematoma can become tender and tense on palpation. Some bleeds lead to a slowly enlarging collection known as a pseudotumour, which causes damage and pain over time.

The fascial compartments around a muscle limit how far blood can spread. When a compartment fills, pressure rises and can compromise nerves and blood vessels. This compartment syndrome is most often described in the lower leg and forearm, and it is a medical emergency because nerve and muscle injury can become irreversible if pressure is not relieved promptly. Bleeds in the neck and throat are a separate airway risk: swelling, difficulty swallowing or breathing, choking, and voice change are recognised warning signs. Retroperitoneal bleeding, especially around the iliopsoas, can cause massive blood loss and is treated as a reason for admission.

Intracranial and other life-threatening bleeding

Intracranial hemorrhage is among the serious bleeds recognised in hemophilia, and internal bleeding into the brain, kidneys and gastrointestinal tract is more likely in severe disease. Emergency guidance treats head or neck trauma as a serious mechanism of injury even when no bleeding is visible, and admission is recommended for bleeding in the head, neck, pharynx or retroperitoneum. Blood in the urine, black or bloody stools, vomiting or coughing up blood, and abdominal swelling are other signs of internal bleeding that require urgent assessment.

In children, the early pattern matters. Severe disease usually becomes apparent in the first months of life, often through bleeding after circumcision, vaccination or minor injury. Some children with severe disease, however, have no obvious bleeding until they begin to crawl and walk, when falls and joint loading increase. Mild and moderate disease may not be recognised until later childhood or adolescence, often at the time of a dental procedure or surgery. Recognising unexplained bruising, prolonged bleeding and bleeding after trauma in children is therefore central to timely diagnosis and to preventing the joint and muscle damage that accumulates from unrecognised episodes.

Diagnosis and laboratory evaluation

Screening and its limits

A bleeding workup usually starts with a platelet count, prothrombin time (PT), activated partial thromboplastin time (aPTT) and fibrinogen. In hemophilia A and B, the aPTT is typically prolonged while PT and platelet count stay normal Hemophilia A or B: aPTT prolonged, PT normal, platelet count normal. The aPTT depends partly on the reagents and instruments a laboratory uses aPTT results are affected by reagents/instruments used by the lab.. A normal result does not exclude mild disease, especially when factor levels sit near the lower reference limit a normal aPTT does not exclude mild hemophilia, particularly when FVIII or FIX activity levels are near the lower limit of normal., and it can also be normal in female carriers the aPTT may yield a normal result in mild cases of hemophilia and in female hemophilia carriers..

A prolonged aPTT has other causes. Factor XI or XII deficiency produces the same pattern, whereas factor VII deficiency and vitamin K deficiency are associated with a prolonged PT Additional factor deficiencies include factor XI and factor XII deficiency (in which case aPTT is elevated), factor VII deficiency, and vitamin K deficiency (in which case PT is elevated)..

Mixing studies

An isolated prolonged aPTT should be followed by a mixing study An isolated prolonged aPTT should prompt further evaluation with a mixing study.. Correction of the aPTT in the mixture points to a factor deficiency, while persistence of the prolongation points to an inhibitor An aPTT that corrects with a mixing study suggests a factor deficiency, whereas an aPTT that does not correct with a mixing study suggests that an inhibitor is present.. Some factor VIII inhibitors act slowly and depend on temperature, so an incubated version (one to two hours at 37 °C) is often needed to detect them Incubated mixing studies (in which a mixture of patient plasma and normal pooled plasma is incubated for 1-2 hours at 37°C before testing) are often necessary to detect factor VIII inhibitors. Lupus anticoagulants and anticoagulant drugs can produce a non-correcting pattern too, so heparin contamination is excluded separately: heparin prolongs the thrombin time, whereas a factor VIII inhibitor does not The presence of heparin can be excluded by treating the plasma with Hepzyme (to inactivate the heparin) and then repeating the APTT, or a Thrombin time performed. Heparin would prolong the Thrombin time but a factor VIII inhibitor would have no affect on the assay.. When acquired hemophilia coexists with a factor deficiency, mixing patterns can overlap, and one case report found a 1:1 percent-correction method the most reliable for separating them Diagnosing coagulopathy is especially challenging when a patient has both acquired hemophilia and coagulation factor deficiency, as mixing test patterns may overlap and lose distinct characteristics. The 1:1 percent correction method proved to be the most reliable approach for distinguishing between deficiency and inhibitor-related patterns.. Where factor assays are not immediately available, a bedside mixing venous clotting time test has been used to triage patients before factor measurement The laboratory investigation of factor VIII and factor IX clotting activity is not immediately available at the hemophilia treatment centers. Therefore, a diagnostic kit of mixing venous clotting time (VCT) to determine the status of hemophilia A and B at bedside followed by the subsequent coagulation factor assay was invented.

Factor assays and inhibitor testing

Definitive diagnosis rests on quantitative factor VIII or factor IX activity Definitive diagnosis of hemophilia largely depends on quantitative measurement of factor VIII and factor IX activity. Low factor VIII activity points to hemophilia A, but it can also arise in von Willebrand disease and acquired hemophilia, so the result must be read alongside von Willebrand factor (VWF) testing and mixing studies Low factor VIII activity largely confirms hemophilia A, though reduced FVIII can also occur in von Willebrand disease and acquired hemophilia A. The result must be interpreted with the clinical history, VWF testing, mixing studies, and inhibitor testing when appropriate.. The same assays grade severity If the Factor VIII level is low, the individual may have Hemophilia A and if the Factor IX level is low, the individual may have Hemophilia B. These clotting factor tests also indicate the severity of the deficiency..

Inhibitors are quantified with the Bethesda assay, in which one Bethesda unit is the amount of inhibitor that neutralises half of a given factor after two hours at 37 °C 1 Bethesda Unit (Bu) corresponds to a quantity of inhibitor in a plasma sample required to neutralize 50% of a single unit of a given factor after incubating for two hours at 37° C.. The Nijmegen modification buffers the control plasma and uses factor-deficient plasma in the control mixture, improving specificity The Nijmegen modification of the Bethesda assay increases the specificity for the measurement of low-titre FVIII inhibitors by buffering the control plasma used in the patient and control mixtures, and it was recommended as the reference method for factor VIII inhibitors This modified Bethesda method was recommended by the International Society on Thrombosis and Haemostasis (ISTH) as the reference method for the FVIII inhibitor test. The assay still has limits at low titres, where false-positive and false-negative rates can be considerable the percentage of false positives and false negatives is high (can be up to 32% and 5% respectively). An inhibitor is suspected when replacement therapy works poorly, with a reduced factor recovery and shortened half-life on pharmacokinetic testing An inhibitor is suspected in case of an inadequate clinical response to replacement therapy, particularly when patients have been responsive previously. In this situation, a poor FVIII recovery and shortened FVIII half-life can be found during a pharmacokinetic (PK) study. The measured titre guides therapy: one hematologist's practical rule, described in a clinical review, is high-dose factor for titres below 5 BU and a bypassing agent above that level If they are low (< 5 Bethesda units [BUs]), give high doses of factor VIII or IX. If they are high (> 5 BUs), give a bypassing agent.

Genetic testing, carrier detection and prenatal diagnosis

Genetic testing has become a confirmatory tool, identifying the disease-causing variant in the F8 or F9 gene through DNA analysis Genetic testing has become an increasingly important component of confirmatory diagnosis in hemophilia by identifying the specific disease-causing variant within the F8 or F9 gene through DNA analysis.. Unlike activity assays, it detects the underlying genetic alteration directly Unlike clotting factor testing, which measures protein activity, molecular testing directly detects the underlying genetic alteration responsible for hemophilia.. Testing can confirm the causal variant in affected individuals, allow targeted testing of a known familial variant, and determine carrier status in girls or women at risk Genetic testing can confirm the presence of the causative F8 or F9 gene variant in affected individuals, allows for targeted testing of documented familial mutations, and can determine carrier status in girls or women at risk.. In families, an affected male is usually tested first to guide testing of relatives An affected male should be the first person tested in a family in order to guide testing of other family members. The World Federation of Hemophilia recommends offering genetic testing to obligate carriers and at-risk female relatives, and investigating the genetic basis of low factor levels in women For all obligate carriers of hemophilia and "at-risk" female relatives of people with hemophilia, the WFH recommends that genetic testing be offered for the previously identified genetic variant in the F8 or F9 gene. For females with low phenotypic coagulation FVIII or FIX levels, the WFH recommends that investigation of the genetic/epigenetic basis of the phenotype be offered..

For prenatal diagnosis, chorionic villus sampling at about 10 to 12 weeks or amniocentesis at 16 to 20 weeks can provide fetal DNA, and when DNA analysis is not possible, fetal blood obtained by fetoscopy at about 20 weeks can be assayed for factor VIII For prenatal testing, carriers with identifiable mutations may undergo chorionic villous sampling at approximately 10 to 12 weeks' gestation or amniocentesis at 16 to 20 weeks' gestation to obtain fetal cells for DNA analysis or linkage studies. When DNA analysis is not possible, fetal blood procured by means of fetoscopy at approximately 20 weeks' gestation may be assayed for the factor VIII level.. Carrier detection in F9 families is harder because factor IX levels are usually normal in carriers The factor IX level is usually normal in carriers of an F9 mutation.. Prenatal diagnosis is recommended for carriers carrying a male fetus, with counselling about the procedural risk to the pregnancy For pregnant females who are carriers of an F8 or F9 variant and are carrying a male fetus, the WFH recommends that prenatal diagnosis (PND) be offered to determine the hemophilia status of the fetus. Genetic counselling should include a discussion of the risk of the PND procedure to the pregnancy..

Inhibitors (alloantibodies)

An inhibitor is an alloantibody: an antibody that the patient's immune system produces against an infused clotting factor it does not recognise as its own. In hemophilia A, the antibodies are usually directed against factor VIII and neutralise its activity. Inhibitors are the most serious treatment-related complication of replacement therapy. They are most common in severe disease, where roughly 30% of patients who have received factor replacement develop them, compared with about 5% of those with mild or moderate disease. Inhibitor formation is tested for with the Bethesda assay, which expresses the antibody titre in Bethesda units (BU) per millilitre. The titre determines both the clinical management and the chances of eliminating the antibody.

Risk arises from a mix of genetic and environmental factors, and the cause is still not fully understood. The most important genetic factor is the type of mutation in the F8 gene, with complete loss-of-function (null) mutations carrying the highest risk in severe hemophilia A. Family history of inhibitors and ethnicity have also been linked to risk, and some reviews point to immune-regulating genes such as CTLA-4 and PTPN22, although these associations are not firmly established. Among environmental factors, the source of the factor product, the age at first exposure, and the intensity of early treatment have all been studied. Dangerous or inflammatory events, such as severe bleeds, surgery, infections and vaccinations, may also trigger an immune response to infused factor. In children with severe disease, inhibitors often appear after roughly 9 to 12 exposure days, typically around the age of one to two years. Older age has also been identified as a risk factor, particularly in non-severe disease, where exposure to factor is spread across a lifetime.

The main clinical consequence is that standard factor concentrate no longer raises factor levels as expected. Bleeds are then managed with bypassing agents, which generate clotting through alternative routes and include activated prothrombin complex concentrate (aPCC) and recombinant activated factor VIIa (rFVIIa). For treating individual bleeds, these agents achieve efficacy of roughly 80–90%, but they are considerably less effective than factor concentrates in preventing bleeds. Patients with inhibitors who receive bypassing-agent prophylaxis have historically recorded substantially more bleeds than patients without inhibitors on optimal factor prophylaxis, who typically have only about 0 to 2 bleeds per year. Repeated bleeding into joints raises the likelihood of early and progressive arthropathy, and the combination of bleeding and bypassing-agent costs makes inhibitor care markedly more expensive. Newer non-factor prophylaxis is discussed elsewhere in this entry.

Immune tolerance induction (ITI) is the standard approach to eradicating inhibitors. It involves giving regular, relatively high doses of factor VIII over a prolonged period, with the aim of retraining the immune system so that the antibody disappears. Success is commonly defined as an inhibitor level below 0.6 BU/mL, factor VIII recovery above 66%, and a factor VIII half-life above six hours. Reported success rates vary between series, generally falling between about 60% and 90%, while roughly one-third of patients fail. Success is more likely when the historical or peak inhibitor titre is low, typically below 100 BU/mL, whereas inhibitors that first appear in the first year of life are associated with a lower chance of success. Low-dose regimens, sometimes combined with immunosuppressive drugs, have been used to reduce cost and burden. ITI is expensive: a typical course may last a year or more and cost over one million US dollars. Economic modelling of a hypothetical five-year-old with high-titre inhibitors nevertheless found that ITI was projected to extend life expectancy and reduce lifetime costs, largely because it removes the need for bypassing agents. Costs are much higher in patients whose ITI fails; in one cohort, the mean cost of the failure group was many times that of patients who achieved complete success.

Acquired hemophilia

Acquired hemophilia A (AHA) is a rare bleeding disorder in which autoantibodies neutralise the body's own factor VIII. It affects people with no previous bleeding history and no family history of hemophilia, which separates it from the congenital form. The antibodies are mostly polyclonal IgG1 and IgG4 molecules. They either block factor VIII's procoagulant activity or accelerate its clearance from the circulation. Because the target is a self-protein, AHA is usually classed among autoimmune disorders and is understood as a breakdown of immune tolerance to factor VIII. Estimates of incidence cluster around 1 to 1.5 cases per million people per year. The rate climbs steeply with age, to roughly 3 to 4 cases per million per year after 60, and men and women are affected in similar numbers.

Associated conditions

Roughly half of cases have no identifiable trigger. Reported idiopathic shares range from about 43 to 52 percent across series, and this proportion is higher among elderly patients. The remaining cases are linked to several categories of underlying condition. Autoimmune diseases, particularly rheumatoid arthritis, systemic lupus erythematosus, Sjögren syndrome and dermatomyositis, account for a notable minority. Malignancy is implicated in about 6 to 18 percent of cases; among haematological tumours, chronic lymphocytic leukaemia is the most frequent, while solid-organ neoplasms also appear more often than expected. Pregnancy is a distinct peak: the risk is raised during pregnancy but is greatest in the postpartum period, extending up to about a year after delivery. Drugs, vaccinations and infections form the remaining group. Some authors also note that repeated transfusion exposure has been linked to inhibitor formation. The appearance of inhibitors is thought to reflect a combination of genetic susceptibility and environmental triggers that disturb immune regulation.

Clinical presentation

AHA differs markedly from congenital hemophilia in its bleeding pattern. Haemarthrosis, the hallmark of severe congenital disease, is unusual. Instead, the dominant features are subcutaneous haematomas, which occur in more than 80 percent of patients, followed by deep intramuscular haematomas in over 40 percent and gastrointestinal bleeding in about 20 percent. Genitourinary, retroperitoneal and other sites are less common. Bleeding can be spontaneous or follow trauma, and surgery, including dental or post-operative bleeding, often reveals the disorder for the first time. Intracranial haemorrhage is rare but usually fatal. The severity of bleeding does not track closely with inhibitor titre, so a modest titre may still be accompanied by serious haemorrhage. Mortality figures vary, with reviews citing between about 8 and 22 percent, and deaths cluster in the first weeks after symptoms appear. Delayed recognition is a major reason for this high case-fatality rate.

Diagnostic clues

The laboratory signature is an isolated prolonged activated partial thromboplastin time with a normal prothrombin time, normal platelet count and normal thrombin time, together with a low factor VIII level. A mixing study is central to the workup. In a 1:1 mix of patient and normal plasma, the aPTT fails to normalise immediately, which points to an inhibitor rather than a simple deficiency. Because factor VIII autoantibodies act in a time- and temperature-dependent way, the aPTT is then repeated after incubation, and further prolongation supports the diagnosis. Heparin, direct oral anticoagulants such as dabigatran, lupus anticoagulant and factor XII deficiency must be excluded first. The diagnosis is confirmed by a reduced factor VIII activity together with a measured inhibitor, quantified by the Bethesda or Nijmegen assay, where titres above 5 BU/mL are generally considered elevated. Early consultation with a haemophilia centre is recommended, because a delay in recognising the condition is a leading driver of harm.

Treatment principles

Management pursues two goals: arresting bleeding and eradicating the inhibitor. Haemostatic therapy uses bypassing agents such as activated prothrombin complex concentrate and recombinant activated factor VII, or porcine or human factor VIII, with treatment tailored to the inhibitor titre and the site and severity of bleeding. Immunosuppression, typically with corticosteroids such as prednisone and with cyclophosphamide or the anti-CD20 monoclonal antibody rituximab, aims to suppress antibody production. Combination immunosuppression appears more effective than single agents, but it also raises the risk of serious infection, which is a leading cause of death in treated patients. In one meta-analysis of 249 cases, complete remission was reached in about 74 percent of patients after a median follow-up of 12 months, with inhibitor-related mortality around 11 percent. Early initiation of immunosuppression once the diagnosis is made is therefore central to outcomes.

Epidemiology and global distribution

Prevalence and the relative frequency of each form

The most cited global estimates come from a meta-analysis that used patient registry data from Australia, Canada, France, Italy, New Zealand, and the United Kingdom—the countries with the most comprehensive registries of hemophilia. Per 100,000 males, the pooled figures are 17.1 for all severities of hemophilia A and 6.0 for severe hemophilia A, against 3.8 and 1.1 for hemophilia B (mean estimated prevalence per 100,000 males was 3.8, and prevalence per 100,000 males for severe hemophilia B was 1.1). Because prevalence describes living patients, it understates how many boys are born with the disorder. The same analysis gives a birth prevalence of 24.6 for all severities of hemophilia A and 9.5 for severe hemophilia A, and 5.0 and 1.5 for hemophilia B.

Extrapolated to the world's male population, these rates imply an estimated 1,125,000 men worldwide have hemophilia, 418,000 of whom have severe hemophilia. Hemophilia A accounts for roughly 80% to 85% of hemophilia cases, while hemophilia B accounts for about 15% to 20%. The earlier global figure of about 400,000 had been based on prevalence in the United States and the global population at the time, which explains much of the upward revision.

Variation in reported prevalence and the role of diagnostic capacity

Estimates differ by method. In the United States, no precise national prevalence estimates for hemophilia exist because of the difficulty identifying cases among persons who receive care from various types of health care providers; state-based studies produced figures of 13.4 and 19.4 per 100,000 males. The US Centers for Disease Control and Prevention's estimate is lower than the meta-analytic value for hemophilia A, at about 12 per 100,000 males. Reported counts, by contrast, track case-finding closely. A global analysis found that globally, prevalence rose from 2.75 in 2014 to 3.75 in 2023 per 100,000 population, with the authors linking the increase to health system strengthening, proactive screening, and registry-based surveillance. Rising reported numbers therefore reflect, in part, better detection rather than a change in the underlying incidence.

Under-diagnosis and disparities between income groups

The gap between expected and identified patients is widest in poorer countries. The prevalence of hemophilia varies globally, with close to 100% of patients diagnosed in high-income countries and as low as 12% diagnosed in lower-income countries. Figures for low-income and lower-middle-income countries vary across sources; one WFH-based summary gives a much lower diagnosis rate, with only 6.3% of those diagnosed in these countries which contribute 59% of the global population. Delays in diagnosis are also long. In the World Federation of Hemophilia's registry, the age of diagnosis of people with hemophilia in low- and low-middle-income countries is delayed by 3 decades compared to patients in upper-middle-income countries and by 4 decades compared to patients in high-income countries. Because the disorder often goes unrecognised until a severe bleed, early mortality of patients before confirmation of their diagnosis is a recognised concern.

Access to treatment follows the same pattern. One analysis reports that only 19% are available to people with haemophilia (PWH) living in LIC and LMICs who constitute 71% of the global population, where 75% of PWH have limited or no treatment access. Mortality is correspondingly higher in these settings, a point the registry summary frames as care provided in low-income countries lags behind high-income countries by up to 40 years.

Trends in life expectancy

Life expectancy has risen substantially over the past several decades, though the rise has not been uniform. In a long-running Dutch cohort, median life expectancy of the cohort increased from 66 years in 1973–1986 to 77 years in 2001–2018, a gain of 11 years. The gains were largest for severe disease: median life expectancy of patients with severe hemophilia was 73 years in 2001–2018, compared with 59 years in the 1992–2001 cohort. The same cohort shows a setback in the 1990s, which the authors attribute to the HIV and hepatitis C epidemics, and the period from 1986 to 2001 was the exception to the otherwise gradual improvement. Earlier UK data point to the same pattern, with median life expectancy in severe hemophilia of 63 years and median life expectancy of 75 years in moderate or mild disease during the study period.

Historical estimates for severe disease are older and vary. One summary states that in the early 1900s, the average life expectancy for a person with severe hemophilia was not more than 13 years, and by the 1960s, it was still less than 20. Another review gives a median life expectancy of roughly 30 years until the 1960s. Despite the gains, excess mortality persists: in the Dutch 1992–2001 cohort the standard mortality ratio (SMR) was 2.3 for the group in total and 5.1 for those with severe hemophilia. Much of this excess is linked to viral infection, since if one excludes individuals with HIV and HCV disease, the SMR improves to 1.2.

History of discovery

Hemophilia is among the longest-recognised inherited disorders. It is recognized as early as the second century. For most of that history, however, its cause was unknown, and the early accounts described a pattern of family bleeding without any explanation of the defect.

Written references to a familial bleeding pattern appear in rabbinic law. A second-century ruling in the Talmud allowed a newborn boy to be spared circumcision when two of his older brothers had died of bleeding after the procedure. Later rabbinic writers extended this reasoning to maternal relatives, recognising that the trait passed through the mother's side of the family. In the tenth century, the Andalusian surgeon Albucasis (935–1013) wrote a description that described a family where males died from bleeding problems. Further families were recorded in the late eighteenth century. Consbruch in 1793 and 1810, Rave in 1796, and Otto in 1803 all described families in which males suffered abnormally prolonged post-traumatic bleeding.

The first modern clinical account is usually credited to John Conrad Otto, a physician in Philadelphia. In 1803 he wrote about a hemorrhagic disposition in certain families, and he recognised that the condition was hereditary and affected males. He traced the disease back through three generations to a woman who had settled near Plymouth, New Hampshire, in 1720. In 1813 John Hay suggested that men affected by the bleeding disorder might pass the trait to their unaffected daughters. Johann Nasse's 1820 review gave the inheritance pattern its enduring form. Nasse's law holds that hemophilia is transmitted entirely by unaffected females to their sons. The name itself dates from 1828, when Johann Lukas Schönlein first documented the word in his dissertation at the University of Zurich; the dissertation was written with his student Friedrich Hopff. The disorder's association with European royalty, especially Queen Victoria's descendants, later gave it the nickname "royal disease." Of her children, one son, Leopold, had Hemophilia B, and two daughters, Alice and Beatrice, were carriers.

Physicians of the nineteenth century mostly located the problem in the vessels. Before the twentieth century, they suspected that people with hemophilia bled easily because their blood vessels were fragile. Practical steps came slowly. A 1840 case of treatment by transfusion was recorded in The Lancet, and in 1893 the prolonged clotting time was first documented in a capillary tube. In the 1930s attention shifted to the blood itself. Doctors then looked at defective platelets as the likely cause.

The decisive step came in 1937. Arthur Patek and F.H.L. Taylor at Harvard published a paper describing an anti-hemophilia globulin found in plasma. The substance could shorten clotting time in patients with hemophilia. In the 1940s, a deficiency in factor VIII was associated with the clotting disorder hemophilia A. Clotting factors were then numbered in order of discovery, and it became known as factor VIII in the 1950s, when a committee of the International Society of Thrombosis and Haemostasis assigned Roman numerals to the known factors. This work placed hemophilia within the study of blood coagulation rather than vascular biology.

The separation of hemophilia A from hemophilia B rested on a simple laboratory experiment. Alfredo Pavlovsky, working in Buenos Aires, observed that mixing plasma from a patient with hemophilia A with plasma from a patient with hemophilia B corrected the clotting time. Sources differ on the date of this observation: one account places it in 1944, when he found that transfusing blood from one hemophiliac to another could correct the clotting defect. His published report from 1947 described blood from some haemophilic patients with a greatly prolonged clotting time that, added to other haemophilic blood, had a coagulant action nearly as effective as normal blood. This was later shown to reflect the blood of people with hemophilia B supplying the clotting factor that was missing in those with hemophilia A.

Hemophilia B itself was described later. It was first described in 1952 in a British medical journal and named after Stephen Christmas, a five-year-old British boy who was the first patient described with the condition. The name Christmas disease persisted for decades. The deficient protein, factor IX, was also known as the Christmas factor. The mixing principle later became the basis for distinguishing the two forms in routine diagnosis, a point developed in the section on laboratory evaluation.

The early twentieth-century transfusion work that followed is covered in the section on blood transfusion and concentrates.

Plasma-derived and recombinant factor concentrates

Replacement therapy began with whole blood products. From the late 1950s through much of the 1960s, fresh frozen plasma was the main treatment, but each unit held only trace amounts of factor VIII and factor IX, so large volumes had to be infused to stop a bleed. Fresh frozen plasma was the mainstay of treatment for hemophilia A and B in the late 1950s and much of the 1960s, but each bag contained only minute quantities of the factors. A more concentrated source followed in 1964, when Judith Pool demonstrated that cryoprecipitation of fresh-frozen plasma could concentrate FVIII in the pellet. By the late 1960s, manufacturers had separated the factors from pooled plasma and packaged them as freeze-dried (lyophilized) powders, with a label stating the amount of factor in each bottle, which allowed more accurate dosing. Because these products could be stored and reconstituted at home, the availability of these concentrates led to home treatment by the early 1970s, greatly changing the lives of people with hemophilia.

The same pooling that made supply possible also created the epidemic of the 1980s. Concentrate lots were assembled from large numbers of donors; one source notes that as many as 20,000 donors may contribute to a single lot of plasma-derived FVIII concentrate, which increased the chance that any one lot contained a pathogen. Before 1985, concentrates were prepared from donors whose HIV status was unknown and were not routinely subjected to viral inactivation. Exposure was heaviest among people with severe disease: from 1979 to 1985, individuals producing less than 1% of normal clotting factor received the largest volumes of concentrate. Estimates of infection vary between sources. One review reports that an estimated 9300 haemophiliacs in the USA, almost half the total, were infected by HIV. Hepatitis infection was even more widespread: in a UK analysis, hepatitis C infection was almost universal after the first treatment with a concentrate prepared from pooled plasma. The first public signal came in 1982, when the Centers for Disease Control and Prevention announced three cases of apparent AIDS in hemophiliacs who had received clotting factors. Some patients had been advised to continue using their products after the link became known, and the financial investment in the product was considered too high to destroy the inventory. The UK inquiry later criticised the authorities for not investing in the expansion of the domestic fractionation service, as they had promised in the 1970s.

Manufacturers responded by building viral inactivation into production. Heat treatment of the freeze-dried product in its final container became the most widely used method, because HIV was found to be susceptible to relatively low temperatures, with sufficient viral kill achievable at 60 °C. Heating in solution proved less reliable at first; one early approach gave very low yields, about eight percent. Heat also reduces activity: in one study, the recoveries fell to 68 and 33 percent at 68 and 80 degrees, temperatures reported to be more effective in viral inactivation. Solvent-detergent treatment was added as a second barrier, and this combination was developed in response to the tragedy to generate safer plasma-derived FVIII products. Even so, heat-treated concentrates did not eliminate every risk: parvovirus B19 transmission occurred in a child who had received a vapor heat-treated prothrombin complex concentrate, and in another child who received dry-heated factor VIII concentrate.

Recombinant products removed human plasma from the production chain altogether. The cloning of the gene for FVIII in 1984 led to recombinant FVIII preparations becoming available in the 1990s. Clinical testing of recombinant FVIII began in June 1988; the reported trial enrolled 107 subjects with hemophilia, 20 of whom had not been treated previously. These products are made from genetically engineered cells instead of donated plasma, which means the source material for each batch is a defined cell line rather than a pool of donors. Recombinant factor IX followed the same path, and the first recombinant coagulation factors were produced in the 1990s, consolidating greater safety and availability. Purification is carried out by chromatographic steps and protein engineering, rather than by fractionating pooled plasma, and the resulting preparations contain no donor-derived plasma proteins. Recombinant FVIII is considered a safer alternative than plasma-derived products, though plasma-derived products remain in use. Current practice involves a wide choice: recent studies showing lower rates of inhibitor development with plasma-derived FVIII products versus recombinant FVIII products have made choosing among the many options more complex.

Prophylaxis versus on-demand treatment

Evidence for scheduled infusion

On-demand treatment gives factor concentrate only after a bleed has started, while prophylaxis gives it on a fixed schedule to keep factor activity above a protective threshold. The strongest early evidence came from a randomized trial in boys under 30 months with severe hemophilia A, which compared scheduled infusions with episodic treatment. Its primary endpoint was the incidence of bone or cartilage damage in the index joints (ankles, knees, elbows), assessed by radiography or MRI. A later analysis of children aged 1 to 7 found that children receiving prophylaxis had fewer hemarthroses than those on on-demand therapy (0.20 vs 0.52 events per patient per month). Annualised joint bleeding rates were roughly 2 to 2.5 in the prophylaxis groups, against about 17 in the on-demand groups. A 2018 meta-analysis of factor VIII studies in patients without inhibitors reported median bleeding rates in on-demand groups that were 67–100% higher than in prophylaxis groups for all bleeds, with pooled estimates favouring prophylaxis.

The benefit is clearest when prophylaxis starts early. In SPINART, a three-year randomized trial in adults with severe hemophilia A and existing joint disease, prophylaxis produced a 94% reduction in bleeding episodes despite severe pre-existing arthropathy, and 35.7% of participants were bleed-free. Yet MRI score changes did not differ between the prophylaxis and on-demand groups, which led the authors to conclude that prophylaxis improves function but must begin before joint bleeding starts in order to prevent arthropathy.

Pharmacokinetics and dosing

The protective effect depends on the factor level maintained between doses. Factor half-life, which determines how quickly circulating activity falls, is the main pharmacokinetic constraint on dosing. Prophylaxis with standard half-life factor VIII concentrates requires two to four intravenous infusions each week. Individual responses also differ. In an individualised arm of one trial, the median dosing interval was 3.5 days, and 30 percent of patients achieved a mean dosing interval of at least 5 days in the last three months of the study. This variation is the rationale for pharmacokinetic-guided dosing, in which the dose and interval are set from each patient's measured factor levels rather than a fixed regimen.

Extended half-life products

Protein engineering has lengthened the circulating time of both factors. Techniques such as fusion to the Fc part of IgG1 or albumin, chemical PEGylation and sequence modification have been used, and factor IX half-life can be extended four- to six-fold. For factor VIII, the half-life of rFVIIIFc was approximately 19.0 hours compared to 12.4 hours for ADVATE. Longer half-life permits less frequent infusion, which is expected to improve adherence. One review notes that extending dosing intervals with extended half-life products could change treatment behaviour, leading to greater adherence to prophylaxis and possibly lower annualised bleeding rates. In the A-LONG trial, median annualised bleeding rates were 1.6 with individualised prophylaxis, 3.6 with weekly prophylaxis and 33.6 with episodic treatment (individualised prophylaxis 1.6; weekly prophylaxis 3.6; episodic treatment 33.6). In an extension study of factor IX, patients who switched to a 21-day regimen had a median annualised spontaneous bleeding rate of 0.0 across all regimens, although the cohort was small, with eleven patients transitioning from a 7-day to a 14-day regimen and then to a 21-day regimen.

Monitoring prophylaxis outcomes

Prophylaxis is usually judged by annualised bleeding rate (ABR) and annualised joint bleeding rate (AJBR). Current guidance recommends regular prophylaxis for patients with severe hemophilia, or for moderate hemophilia with a severe bleeding phenotype, defined as an ABR of at least 5 bleeds per year or an AJBR of at least 3 joint bleeds per year. Joint structure is tracked with the Hemophilia Joint Health Score (HJHS). Its total score runs from 0 to 124 across the six most commonly involved joints, and it is a physical examination tool sensitive enough to pick up subtle early signs of joint damage. It was developed to detect early joint changes in boys aged 4 to 18, and its use in adults is still being validated. Ultrasound is increasingly used alongside it: in one three-year study, synovitis detection decreased from 12.9% to 1.6%.

Bleeding counts and clinical scores do not capture everything. In one long-term cohort, joint health deteriorated over 5 to 10 years in 37% of patients with moderate or severe hemophilia and low joint bleeding rates, which indicates that monitoring of joint status must continue even when bleeding is well controlled.

Non-factor therapies

Non-factor therapies prevent bleeding without supplying the missing clotting protein. Each one works on a different point in the coagulation pathway, and most are given by subcutaneous injection.

Emicizumab

Emicizumab is a humanized bispecific antibody that brings together activated factor IX and factor X, so it can take over the cofactor function that factor VIII normally performs. It was developed as an FVIII mimetic to address unmet needs in hemophilia A. The first approval came from the US Food and Drug Administration on November 16, 2017, for routine prophylaxis to prevent or reduce the frequency of bleeding episodes in adult and pediatric patients with hemophilia A with factor VIII inhibitors. In October 2018 the FDA expanded the approval to all adult and pediatric patients, newborn and older, regardless of inhibitor status. The European Medicines Agency granted its initial authorisation in February 2018 (initial authorisation), and Japan approved the drug in the same year. Labels have been revised since then; in the EU, for example, the indication was broadened in 2023 to include people without inhibitors who have severe disease or a severe bleeding phenotype with moderate disease. By March 2023, more than 20,000 people had been treated worldwide.

In the pivotal HAVEN 1 trial, the annualized rate of bleeds requiring factor treatment fell from 23.3 to 2.9 among patients receiving prophylaxis, an 87% reduction. Patient-reported measures of hemophilia symptoms and physical function also improved. Emicizumab is generally regarded as the first subcutaneously administered non-replacement agent for continuous prophylaxis, and it is effective in patients both with and without inhibitors.

Subcutaneous regimens, quality of life and adherence

Non-factor agents differ mainly in how often they must be injected. Emicizumab has been studied at several schedules: in HAVEN 3, weekly dosing of 1.5 mg/kg and dosing every two weeks at 3 mg/kg both produced large reductions in bleeding, and in HAVEN 4 a 6 mg/kg dose given every four weeks was evaluated. The weekly and biweekly arms experienced 96-percent and 97-percent reductions in their annualized bleeding rates. Fitusiran, an antithrombin-lowering siRNA, can be dosed once every two months, and its maker describes it as offering consistent protection with as few as six injections per year. Concizumab is given once daily, with a loading dose of 1 mg/kg on day one followed by a daily maintenance dose of 0.2 mg/kg, adjusted on the basis of plasma concentration monitoring. Marstacimab was studied in a weekly subcutaneous regimen in the BASIS trial, where bleeding rates fell in patients with inhibitors without unexpected side effects.

Dosing frequency matters because many people with hemophilia have to balance tight bleed control against the burden of treatment. Patients often feel they must choose between effective bleed control and convenient dosing schedules. Fewer injections can therefore improve adherence and quality of life, although long-term outcome data for the less frequent schedules are still accumulating.

Rebalancing agents: antithrombin and TFPI

Natural anticoagulants restrain thrombin generation, and the newer agents are designed to lower that restraint rather than add clotting factor. Two targets have reached clinical use. The first is antithrombin. Fitusiran is an antithrombin-directed small interfering RNA that reduces hepatic antithrombin synthesis. The US FDA approved it on March 28, 2025, for routine prophylaxis in patients aged 12 and older with hemophilia A or B, with or without inhibitors. Dosing is titrated to keep antithrombin activity between 15% and 35%, with monitoring by an FDA-cleared assay. In the ATLAS program, patients without inhibitors had a 71% reduction in annualized bleeding rates compared with on-demand factor concentrate.

The second target is tissue factor pathway inhibitor (TFPI), which normally limits factor Xa production by the tissue factor–factor VIIa complex. Concizumab and marstacimab are monoclonal antibodies that both bind the Kunitz-2 domain of TFPI, and although the two share an identical mechanism of action, they show distinct pharmacokinetic properties. Marstacimab is an IgG1 monoclonal antibody designed to target TFPI. Its approval began with patients without inhibitors (approved for prophylactic use in individuals with hemophilia A or B without inhibitors), and it was later approved for patients with or without inhibitors. Concizumab is approved in the EU for once-daily subcutaneous prophylaxis in patients with hemophilia A or B, with or without inhibitors.

Safety and monitoring

The safety profile of these agents remains the main open question. All of these agents have been associated with serious thrombotic and/or thromboembolic adverse events in clinical trials. Preclinical work in a primate venous stasis model found that thrombus formation with emicizumab did not markedly exceed thrombus formation seen with FVIIa, aPCC, or FVIII. Real-world data for concizumab remain very limited, and investigators have called for follow-up studies to assess long-term safety and efficacy. Because the drugs act on the balance of the coagulation system, monitoring is tied to specific biomarkers: antithrombin levels for fitusiran and drug concentrations for concizumab, which is first measured after a stable maintenance period of four to eight weeks.

Gene therapy

Gene therapy for hemophilia uses adeno-associated virus (AAV) vectors to deliver a functional F8 or F9 gene to hepatocytes. The liver is the target because it is the main site of clotting factor synthesis. In the standard design, a single infusion of AAV particles carrying complementary DNA for factor VIII or factor IX leads to production of the protein in liver cells, which can bring long-term protection against bleeding. Because the vector is a viral carrier and the inserted sequence is a gene copy rather than a corrected endogenous gene, the transferred sequence is called a transgene, and the level of factor it produces depends on how that transgene is expressed over time.

Approvals began with hemophilia B. Etranacogene dezaparvovec (Hemgenix) received FDA approval in November 2022 for adults with hemophilia B. Fidanacogene elaparvovec (Beqvez) was approved by the FDA in 2024. For hemophilia A, valoctocogene roxaparvovec (Roctavian) received conditional approval from the European Commission in August 2022, and FDA approval followed in June 2023. In April 2025, China's NMPA approved dalnacogene ponparvovec (BBM-H901) as the first gene therapy for adults with moderate to severe hemophilia B in China. Valoctocogene roxaparvovec, etranacogene dezaparvovec and fidanacogene elaparvovec are approved only for adults without factor inhibitors, and the current inhibitor status of patients is therefore a central eligibility criterion. Earlier trials also excluded most people with a history of inhibitors.

Selection criteria are based on several factors. The FDA-approved Roctavian label requires that patients be without pre-existing antibodies to adeno-associated virus serotype 5, detected by an FDA-approved test, because such antibodies may render the gene therapy less effective or ineffective. Liver health is another gatekeeper. Most hemophilia gene therapy trials exclude patients with suboptimal liver conditions such as fibrosis or active viral hepatitis, yet poor liver function and chronic viral hepatitis are not uncommon in people with hemophilia. Pre-treatment assessment therefore includes liver enzyme tests and imaging, such as a liver ultrasound and elastography.

Durability is uneven between the two factors. Factor IX expression is generally stable over time, whereas factor VIII levels often decline. The decline is commonly described in hemophilia A studies. Factor VIII activity has been reported to fall from an initial post-treatment range of 40–60% to 15–20% over 2 to 7 years, and after 2–4 years, many individuals in the Roctavian pivotal study had factor VIII activity below 5%. Proposed causes include immunological factors, including vector- and transgene-directed immune responses, and non-immunological factors such as hepatocyte turnover and epigenetic regulation of transgene expression. Variation between individuals is also substantial, since clinical responses are heterogeneous, with variability in the level and durability of transgene expression among treated individuals. Long-term projections for hemophilia B are based on modelling rather than observed data. One analysis of etranacogene dezaparvovec predicted that over 80% of people would not need factor replacement products 25.5 years after treatment.

Liver monitoring is a central part of follow-up. Immune responses against transduced hepatocytes can cause liver inflammation and reduced FVIII activity. Transient increases in liver enzymes are the most common adverse effect, and these are usually managed with corticosteroids. For Hemgenix, patients are monitored and tested weekly for the first three months after infusion, checking liver enzyme and factor IX activity levels, and corticosteroids may be given if liver enzyme levels are high to reduce inflammation. For hemophilia A, routine imaging screening for hepatocellular carcinoma, preferably every 6 months, is recommended in all recipients, although the long-term risk of hepatocellular carcinoma associated with gene therapy is unknown.

Costs and logistics limit access. One review lists the price of Roctavian at $2.9 million. Because the procedure requires eligibility testing, a single infusion, and weekly early monitoring with the possibility of immunosuppression, delivery is confined to specialised treatment centres. Long-term surveillance of liver health adds further demands on those centres. Durability of expression remains variable and uncertain, and transient liver enzyme elevations require monitoring and immunosuppressive management.

Long-term complications and comorbidities

Hemophilic arthropathy

Repeated bleeding into the same joint is the most consequential long-term problem in hemophilia. Blood left in a joint sets off a sequence in which the synovial lining swells, its vessels multiply, and inflammatory cells accumulate; this early stage is called synovitis, which is characterized by synovial hypertrophy, migration of inflammatory cells, and a high degree of neo-angiogenesis with subsequent bleeding. Iron released from degraded hemoglobin keeps the inflammation going: joint bleeding favors iron release from hemoglobin, thus inducing a chronic inflammatory process mediated by cytokines and pro-angiogenic factors. Cartilage and subchondral bone are then damaged, although the pathogenic mechanisms and molecular pathways by which blood in the joint cavity causes articular cartilage and subchondral bone destruction have yet to be fully elucidated.

The course is not simply proportional to the number of bleeds. Structural joint damage may occur following just a few (or no) hemarthroses in persons predisposed to arthropathy, while others accumulate damage only after many episodes. Persistent bleeding is especially harmful: repeated attacks, or hemorrhage lasting more than six months, can lead to chronic disabling arthropathy. Bone and limb development may also be affected, since repeated bleeding into joints can result in abnormalities in both bone growth and limb length. The resulting disease resembles degenerative and inflammatory arthritis in part, as it appears to have similarities with the degenerative joint damage that occurs in osteoarthritis and the inflammatory processes associated with rheumatoid arthritis. In the literature this is termed hemophilic arthropathy, and it is among the most common reasons for morbidity in people with severe disease.

Chronic pain, mobility and rehabilitation

Pain and restricted motion are the symptoms that most directly erode daily function. Recurrent joint bleeding causes synovial proliferation and inflammation (haemophilic synovitis) that contribute to end-stage degeneration (haemophilic arthropathy); with pain and limitation of motion severely affecting patients' quality of life. Immobility compounds the problem, because patients are also at increased risk of developing osteoporosis as a result of prolonged periods of immobility and reduced range of joint movement.

Management is therefore built around prevention and restoration. Effective prevention and management of haemophilic arthropathy includes the use of early, aggressive prophylaxis with factor replacement therapies, as well as elective procedures, including restorative physical therapy, analgesia, aspiration, synovectomy, and orthopaedic surgery. Care is best delivered by a team; optimal treatment of patients with haemophilia requires a multidisciplinary team comprising a haematologist, physiotherapist, orthopaedic practitioner, rehabilitation physician, occupational therapist, psychologist, social workers, and nurses. Newer experimental approaches target the inflammatory drivers directly. One registered trial is testing intra-articular bevacizumab for recurrent hemarthrosis with chronic synovitis; bevacizumab is a recombinant humanized monoclonal antibody that specifically inhibits VEGF.

Infections acquired through historical transfusions

Chronic viral infection is the second major legacy of the past. Before effective viral inactivation, plasma-derived products transmitted blood-borne viruses. Early generation clotting factor concentrates, without viral inactivation procedures in the manufacturing process, led to an increased risk of transmission of blood-borne viral infections, mainly due to hepatitis C virus and human immunodeficiency virus. Virtually all persons with hemophilia who received clotting factor concentrates before effective viral inactivation techniques were implemented by the mid 1980's became infected with hepatitis C virus. Of those infected, approximately 80% are estimated to have developed chronic infections, and 20% of these are estimated to progress to cirrhosis.

Liver disease risk rises with time and with co-infection. In one cohort of 1816 HCV-seropositive hemophilic patients at 16 centers, ESLD risk increased steeply with age in both groups. Progression is also linked to a longer history of infection, presence of HCV genotype 1, HIV coinfection, and lifestyle characteristics such as alcohol abuse and the presence of the metabolic syndrome. Treatment has changed the picture: it was not until the approval of entecavir and tenofovir (2005-2008) against the B virus, and of direct-acting antiviral agents (2015) against the C virus, that a truly effective treatment became available for liver disease. Hepatitis B and C are not the only issues; because many patients take drugs for these infections, medications used for other comorbidities, such as blood-borne viral infections, are frequently nephrotoxic and hepatotoxic, which adds renal and hepatic monitoring to routine care.

Age-related comorbidities

Improved care has shifted attention toward conditions of later life. Life expectancy has risen sharply: the life expectancy of patients with hemophilia increased from 11 years only in the 20th century, to 55–63 years following the introduction of factor concentrates, to over 65 years in recent reports from the Netherlands and Italy. The proportion of older patients is still small in some centres, but it is growing. Currently, 2% of hemophilia A and B patients surveyed in US comprehensive hemophilia treatment centers are 65 years of age or older and 15% are 45 years or older.

Cardiovascular risk presents a mixed picture. People with hemophilia experience lower CVD-related mortality compared to the general population, yet the prevalence of cardiovascular risk factors such as hypertension and obesity is significantly higher. Overt coronary artery disease also becomes more common with age, from 0.05% in those under 30 to 15.2% in those 60 years or older. Among older people with severe haemophilia A in trial cohorts, the burden is substantial: 72.9% had ≥1 CV risk factor and 74.0% had HIV and/or HCV infection. Haemorrhage risk also changes with age; hypertension together with the clotting factor deficiency causes an increased risk of intracerebral bleeding, and many of the complications of hemophilia, including intracranial hemorrhage, joint disease, and inhibitor development, increase with increasing age. Malignancy is another concern, since the incidence of non-virus related cancer seems to be higher in PWH than the general population. Managing these conditions requires coordination between haemostatic and cardiovascular care, with atherosclerotic disease in particular needing individualised decisions about antithrombotic therapy.

Care organisation, access, and health economics

Multidisciplinary treatment centres

Care for hemophilia is usually organised around specialised hemophilia treatment centres (HTCs). In the United States, the core team typically includes a pediatric or adult hematologist, a nurse coordinator, a social worker, and a physical therapist, while orthopedists, dentists, genetic counselors and dietitians are reached on site or through referral. The World Federation of Hemophilia (WFH) lists a medical director, nurse coordinator, musculoskeletal specialists, a medical laboratory specialist, a psychosocial specialist, and patients and family caregivers as core members, though the division of roles differs between centres depending on staff expertise. The WFH also advises a yearly review covering blood counts, joint and muscle health and quality of life, every six months for children. Smaller centres and family physicians can manage some complications, but they work in close consultation with the specialist centre, which matters most for patients who live far from one.

Surgery illustrates why the team model matters. Expert guidance holds that planning elective operations is most effective with the involvement of a specialist and experienced multidisciplinary team, and that surgery in patients with inhibitors requires even more preparation (see antibody-related sections). Physiotherapists supervise the gradual return to activity after a joint or muscle bleed, and social workers have a growing counselling role. A survey of United States centre social workers found that their most prominent actual role was case management, while they judged counselling to be the most important role for their clients; barriers to the ideal role included lack of input, insufficient budgeted time and inadequate training.

Global access through national programmes and patient organisations

Access is sharply uneven. The WFH, founded in 1963, now includes national member organisations from 147 countries, and it works through its own organisational network to support national systems. Its Humanitarian Aid Program began in 1996 with donations of clotting factor that were irregular and therefore limited the programme's effectiveness to responding to catastrophic bleeding events. A 2014 agreement with two pharmaceutical companies for sustained donations of extended half-life factor concentrates changed that: since 2018 the programme has received more than 200 million IU of clotting factor concentrates per year. The WFH has also coordinated donations of emicizumab with private laboratories, though the authors of one review argue that governments, physicians and patient organisations must also work to widen access.

Diagnosis is the first barrier. Diagnosis rates range from close to 100% in high-income countries to as low as 12% in lower-income countries, which means that many patients in low-resource settings never reach a treatment centre at all. Where diagnosis is possible, starting prophylaxis early remains uncommon; one review notes the absence of adequate care such as starting prophylaxis before the age of three, as recommended by WFH guidelines. Low-dose prophylaxis is common in lower-resource settings, and it reduces bleeding, although its effect is smaller than that of standard regimens.

Cost-effectiveness and the strain on health budgets

Conventional replacement therapy is costly to sustain. A US analysis reported that prophylaxis therapy requires several infusions per week, and the cost can range from $300,000 to $776,000 per year. Non-factor products such as emicizumab shift this cost but do not remove it, and comparisons with them depend heavily on local prices and assumptions.

Gene therapy has raised the question of how a single, very expensive treatment should be valued. The debate around its cost-effectiveness has largely centred on its price, which is €1–€3.5 million per individual. The list prices of the first two approved products are 2.9 million and 3.5 million US dollars. Economic models give different answers. A St. Jude analysis of severe haemophilia B estimated that, using a standard half-life factor IX comparator, gene therapy was cost-effective in 92% of simulations at a willingness-to-pay threshold of $150,000 per QALY, but the same team's per-patient cost for manufacture, distribution and five years of follow-up was far lower than the list price, at $87,198. A systematic review concluded that the value of gene therapy depends on the data used in the economic evaluation and on how long the treatment effect is sustained, which is currently uncertain. A Brazilian analysis of etranacogene dezaparvovec estimated an incremental cost-effectiveness ratio of USD 261,436 per bleeding event avoided. Delivery costs also matter: a Dutch microcosting study estimated average delivery costs of about €28,696 for hemophilia A and €20,511 for hemophilia B, excluding some pretreatment labour. Manufacturers argue that their products save money over a lifetime, a claim based on company estimates rather than independent analyses.

Psychosocial support, education and daily life

Psychosocial care is a component of the core team rather than an optional extra. Treatment centres commonly provide education on self-infusion and health management, individual and family counselling, and family screening with genetic counselling. Living with a lifelong condition that requires frequent treatment, or that can cause sudden pain and disability, affects education, work, family roles and mental health. Support is therefore directed not only at infusions and joint care but also at the patient's wider circumstances, including caregivers, who the WFH counts among the core members of the care team.

Research frontiers and unresolved questions

Predicting inhibitor development

Inhibitors remain among the hardest problems in the field, and a central goal is to identify, before treatment starts, which patients will form neutralising antibodies against infused factor. Immune tolerance induction (ITI) is demanding and successful in about 70% of people. Success rates differ by form: ITI eliminates an inhibitor for about 70% of hemophilia A subjects and about 30% of hemophilia B subjects, though the timeline for removal varies markedly. A multicentre cohort of 206 children with severe hemophilia A found that a clinical model could estimate the chance of success. Its four predictors were the cumulative number of FVIII exposure days at inhibitor development, peak inhibitor titer, ethnicity, and F8 mutation type. Peak titre has been the most influential single variable. In a separate analysis of 237 patients, the discriminatory power of the model was 0.87 with the historical peak titer and 0.69 without.

Immunogenetic and immunological markers are under study as well. Genetic factors and biomarkers associated with primary tolerance included HLA allele subtypes and single nucleotide polymorphisms in immune response genes such as IL-10, TNFA, and CTLA-4. Machine-learning approaches have also been applied, but their reported accuracy must be read cautiously, because predicting inhibitor development presents significant computational challenges due to data imbalance, skewed data, and inadequate data sanitization. The practical aim is a validated tool that lets clinicians and families weigh the burden of ITI against the risk of inhibitors before exposure begins.

Durability of gene therapy and loss of transgene expression

Gene therapy based on adeno-associated viral vectors has shown long-term benefit, yet durability is uneven. Long-term follow-up from pivotal trials has documented sustained factor expression for over 10 years in hemophilia B but up to 5 years in hemophilia A, though inter-individual variability remains substantial. FIX expression is generally stable over time, whereas FVIII levels often decline. In one hemophilia A trial, average transgene expression decreased by approximately half from year 1 to year 2 after vector administration, and this decrease continued in the 4-year follow-up data.

Research has shifted toward the mechanisms behind these declines. One study identifies transcriptional efficiency, rather than vector genome copy number, as a primary determinant of variability and durability in treatment response. Other contributors include vector genome loss, hepatocyte turnover, immune responses, and cellular stress, along with epigenetic modifications. Children add a further problem, since higher hepatocyte turnover during growth leads to loss of episomal gene expression over time.

The management of patients who lose expression is an open question. Re-dosing is constrained because AAV vectors do not support efficient re-administration due to the development of anti-capsid neutralizing antibodies. Some patients may need prolonged immunosuppression, and many patients require prolonged immunosuppression with corticosteroids. Proposed strategies include next-generation vector design, codon optimisation, gene editing, and alternative delivery platforms. Preclinical work in male mice has examined an enhanced-function FVIII variant that normalizes hemostasis at below-normal expression without evidence of prothrombotic risk, but this remains experimental.

Next-generation non-factor agents and combination regimens

Non-factor products have expanded the options considerably. Fitusiran, an antithrombin-lowering siRNA approved in 2024, works by degrading antithrombin to enhance clot formation. Concizumab and marstacimab target a different inhibitor, and by blocking TFPI or lowering the concentration of antithrombin, these agents restore thrombin generation even in the absence of normal FVIII or FIX. Pipeline candidates extend this logic: Serpin PC depletes the pool of activated protein C, while several candidate molecules reduce the pool of protein S.

Several gaps remain. Safety is a continuing concern, since fitusiran carries a boxed warning for thrombotic events and gallbladder disease. Practical questions are also unresolved. There is no data on the optimal approach to safely switch from a FVIII mimetic or other rebalancing agent to concizumab. Optimal surgical management of patients on concizumab is not known. How these agents should be sequenced or combined with each other, with factor replacement, or with gene therapy is still largely undefined.

Equity of access across countries

Advanced therapies raise the question of who will receive them. Gene therapy faces challenges such as high cost, uncertain long-term durability, potential liver toxicity, and exclusion of pediatric patients, those with inhibitors towards FVIII or FIX, and those with pre-existing antibodies to AAV, which restrict its reach even within high-income systems. Non-factor agents are expected to play an important role for many patients with hemophilia. Their value depends on whether national programmes can fund them and whether treatment centres can deliver them safely. Patients with inhibitors, a group that has long been underserved, illustrate the gap: for hemophilia B with inhibitors, 2025 has finally brought effective treatment options after a long period with few choices. Translating these advances into treatment available beyond well-resourced centres remains one of the field's least resolved challenges.

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