Hematocrit is the fraction of whole-blood volume occupied by red blood cells. It is reported as a percentage or as a volume fraction in liters per liter: a hematocrit of 45% is equivalent to 0.45 L/L. Commonly included in a complete blood count, it helps assess red-cell status and changes in blood concentration. It reflects both the volume of circulating red cells and the volume of the surrounding plasma, rather than directly measuring the total number of red cells in the body.
Definition and physiological meaning
Hematocrit is defined by the relationship
Although its name refers broadly to blood cells, routine hematocrit excludes white blood cells and platelets. These normally occupy only a small fraction of blood volume.
Red cells contain hemoglobin, the protein responsible for carrying most of the blood’s oxygen. Hematocrit therefore relates to oxygen-carrying capacity, but it is not a measurement of hemoglobin concentration, oxygen saturation, or oxygen delivery to tissues. Oxygen delivery also depends on blood flow and other physiological factors.
A higher hematocrit generally increases blood viscosity, although the relationship is nonlinear and is affected by red-cell deformability, aggregation, and flow conditions.
Measurement
Centrifuged packed cell volume
In the microhematocrit method, anticoagulated blood is placed in a narrow tube and centrifuged. Red cells form a packed layer at the bottom, plasma remains above, and a thin buffy coat containing white cells and platelets lies between them. The packed red-cell column is measured relative to the total blood column.
This measurement is commonly called packed cell volume. A small amount of plasma remains trapped between the packed cells, so the measured packed volume is not precisely identical to the volume of the red cells alone. Centrifugation conditions and reading technique also affect the result.
Automated hematocrit
Many automated blood analyzers calculate hematocrit from the red-cell count and mean corpuscular volume (MCV), the average volume of an individual red cell:
For example, a red-cell count of and an MCV of give a hematocrit of , or 45%.
Some point-of-care instruments estimate hematocrit using other physical properties, such as electrical conductivity. Results from different methods may not be interchangeable under all conditions.
Reference intervals and variation
Reference intervals vary with laboratory method, age, sex, altitude, and physiological state. Adult values commonly lie around 40–50%, but this is not a universal reference interval or diagnostic threshold. Newborns generally have higher values than older children and adults.
During pregnancy, plasma volume usually increases proportionally more than red-cell volume, lowering hematocrit through physiological dilution. Living at high altitude can increase red-cell production in response to reduced oxygen availability.
Fluid balance is especially important. A change in hematocrit can occur without a corresponding change in total red-cell mass:
- Hemoconcentration occurs when plasma volume decreases, increasing the red-cell fraction.
- Hemodilution occurs when plasma volume increases, decreasing the red-cell fraction.
Consequently, hematocrit is a concentration-related measurement, not a direct measurement of total circulating red-cell mass.
Clinical interpretation
Low hematocrit
A low hematocrit commonly accompanies anemia. Mechanisms include blood loss, reduced production of red cells, and increased red-cell destruction, known as hemolysis.
Reduced production may occur in iron-deficiency anemia, vitamin deficiencies, disorders of the bone marrow, or chronic kidney disease. The kidneys normally contribute to the regulation of red-cell production through erythropoietin.
Plasma-volume expansion can also lower hematocrit without a proportionate loss of red cells. Hematocrit alone does not establish the cause of a low result.
High hematocrit
An elevated hematocrit may reflect either an increase in red-cell mass or a reduction in plasma volume. Dehydration is a common cause of relative elevation.
An absolute increase in red-cell mass can occur in response to sustained low oxygen availability, increased erythropoietin production, or a primary disorder of red-cell production. Distinguishing these mechanisms requires information beyond hematocrit itself.
Relationship to other blood measurements
Hematocrit is interpreted alongside hemoglobin concentration, red-cell count, MCV, and other findings. A traditional approximation is that hematocrit expressed as a percentage is about three times hemoglobin concentration expressed in grams per deciliter. This relationship is only a rough consistency check and becomes unreliable when red-cell size or hemoglobin concentration within cells is abnormal.
Limitations and sources of error
Hematocrit may initially remain near its previous value after acute whole-blood loss because red cells and plasma are lost together. It often decreases later as fluid shifts or administered fluids expand plasma volume. An early result therefore does not reliably indicate the extent of acute bleeding.
Sampling and handling also matter. Prolonged tourniquet application, posture, capillary sampling technique, contamination by intravenous fluids, inadequate mixing, and delayed analysis can alter results. Red-cell swelling during storage can increase MCV and thus affect a calculated hematocrit.
Red-cell clumping and other analytical interferences may produce inconsistent automated measurements. Because hematocrit depends on both cellular characteristics and plasma volume, a single result cannot independently establish a diagnosis or determine the need for blood transfusion.