A blood test is an examination of a sample of blood that measures its cells, dissolved chemicals, proteins or other constituents. Clinicians use the results to judge a person's health, to identify disease, and to follow how a condition responds to treatment. Blood tests are a central part of medicine, and they are performed in hospitals, clinics, and commercial laboratories.
Purposes
Clinicians use blood testing for three broad purposes: screening, diagnosis, and monitoring. Broadly, the roles of blood testing in patient care include screening, diagnosis, and monitoring.
Screening looks for disease in people who have no symptoms. Screening tests are ordered to check for medical conditions in people without symptoms. A goal of screening tests is to find medical problems early when treatment may be more effective. Blood tests are used for screening in conditions such as certain cancer types, high cholesterol, and diabetes-mellitus, as well as some sexually transmitted infections (certain types of cancer, high cholesterol, diabetes, and sexually transmitted diseases).
Diagnosis uses blood tests to help confirm or rule out a suspected condition. The conditions for which doctors order blood tests include allergies, anemia, blood clotting problems, heart disease, infection, inflammation, kidney disease, and thyroid disorders, among others.
Monitoring repeats tests over time. Monitoring tests can be used to check a person's health over time and assess whether medical treatment is working. Periodic monitoring tests may be used to keep track of chronic diseases like diabetes or heart disease or to regularly assess the amount of a specific medicine in your body.
Sample collection
The most common method is venipuncture, in which a needle draws blood from a vein. Venipuncture is the process of obtaining intravenous access for the purpose of venous blood sampling or intravenous therapy. It is usually performed by trained staff such as phlebotomists, nurses and laboratory scientists. Blood is typically drawn from the inner elbow, and the blood is collected into sealed tubes or vials. Most blood collection in the United States and the United Kingdom is done with an evacuated tube system.
A second method is the finger stick, which uses a small puncture to obtain a few drops of blood from a capillary. The choice between the two depends on how much blood a test requires. The method used depends on how much blood is needed for the test you are having.
Samples are handled differently depending on the analysis. Cell counts are generally performed on whole blood, while immunoassays and many chemical tests use serum or plasma, the fluid portion that remains after the cells are separated. Because blood is made up of a fluid component and cellular components, a single draw can supply material for many different analyses.
Common types of test
Blood tests fall into several broad groups. Some measure a set of related items at once, while others look for one specific substance.
Blood cell tests
The hematology tests examine the cells and platelets. The complete blood count (CBC) is the most frequently used example. It counts red blood cells, white blood cells and platelets. A CBC test includes a hematocrit test that measures the percentage of red blood cells in your blood. It also measures hemoglobin levels. The CBC is used to look for anemia, bleeding disorders, and blood clotting disorders. The counted cells include red blood cells, white blood cells, and platelets, and the hemoglobin content is a separate measurement.
Chemistry and electrolyte tests
A basic metabolic panel (BMP) measures several dissolved substances. It typically includes a blood glucose test, which screens for diabetes, along with calcium and urea measurements. Any imbalance in these levels may indicate problems with vital organs like the kidneys, lungs, or heart. Related panels, such as the comprehensive metabolic panel, add tests for liver function.
Lipid panels measure fats in the blood. They report two kinds of cholesterol: high-density lipoprotein, often called "good" cholesterol, and low-density lipoprotein, often called "bad" cholesterol. Low-density lipoprotein can form plaque that clogs blood vessels and increases the risk of heart disease. These measurements are used in assessing cardiovascular-disease risk.
Liver tests measure several enzymes and proteins. Their levels can rise or fall when the liver is damaged. The test looks at several different enzymes and proteins, which can be higher or lower than normal if the liver is damaged.
Hormone tests
Thyroid panels assess how the thyroid gland is working by measuring the hormones it produces and the signaling hormone that regulates it. Abnormal levels of hormones in the body can indicate conditions such as thyroid growth disorders, low protein levels, and abnormal levels of estrogen or testosterone.
Clotting tests
Coagulation panels measure how quickly blood clots and how effectively it does so. Clotting is important for wound healing; however, if a clot forms in an artery or vein, it can block the blood flow to vital organs and cause health issues. Results can help detect conditions including bleeding disorders and thrombosis, and they can reveal certain vitamin deficiencies.
Cardiac and inflammation markers
Troponin, a protein released into the bloodstream when heart muscle is damaged, is commonly measured in people with chest pain. Troponin is a protein that's released into your bloodstream when the heart muscle is damaged. Creatine kinase, another enzyme, is used alongside troponin to assess heart damage. Natriuretic peptide (BNP) levels can help diagnose heart failure and indicate its severity.
Inflammation is often assessed with C-reactive protein (CRP), which the liver produces when tissues are inflamed. Markers of this kind are part of the broader study of inflammation and are used as biomarkers.
Blood gas tests
Arterial blood gas tests are drawn from an artery, typically in the wrist, to measure acid-base balance and the levels of oxygen and carbon dioxide. They are most often used in emergency departments and intensive care units, particularly when a person has breathing difficulties.
Blood typing and immune testing
Blood compatibility testing relies on the reactions between blood group antigens and antibodies. When an antibody binds to an antigen on a red blood cell surface, the cells may clump together, a process called agglutination. Techniques that rely on antigen-antibody reactions are termed serologic methods, and several such methods are available, ranging from manual testing using test tubes or slides to fully automated systems. Blood tests can also detect autoimmune disorders, in which the immune-system mistakenly attacks the body's own tissues, and can show whether the immune system is responding to infection.
Interpreting results
A result is judged against a reference range, which is the span of values typically seen in healthy people. Reference ranges differ between laboratories, because each laboratory uses its own equipment, methods, and population data. Always refer to the reference ranges established by the laboratory performing the test before evaluating patient results.
Ranges also vary with personal characteristics. Many factors can affect test results, including age, sex, race, current medications, medical history, overall health, diet, adherence to test-prep instructions, and variations in lab techniques. For example, the typical white blood cell count for adults is commonly given as about 4,300 to 10,800 cells per cubic millimeter, but the exact limits vary by source and laboratory. An abnormal result is a signal for further evaluation, not a diagnosis by itself.
Factors affecting accuracy
Preparation requirements depend on the test. No preparation is needed for a blood test in many cases, but some may require the patient to fast or stop taking certain medications prior to the procedure. Medications are a particular concern: some medications can elevate or lower levels of certain substances in the bloodstream, interfering with the accuracy of the blood test.
Timing can also matter. A systematic review noted that the CBC is commonly ordered yet its reference values do not take the time of day into account, and that several components show daily variation.
Results from a blood test are also limited by sampling and handling. Errors during collection, such as improper filling of tubes, can produce misleading laboratory results, which is why standard procedures are used.
References
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- Your guide to 11 different types of blood tests - BHFbhf.org.uk
- Blood compatibility testingen.wikipedia.org
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