What is a Hematocrit Test?

A hematocrit test measures the proportion of the volume of the blood that consists of red blood cells. This is traditionally done by centrifugation. It is commonly ordered, by doctors, as part of a complete blood count (CBC).

The centrifugation is done in thin capillary tubes. The blood separates into two layers. The layer of red blood cells at the bottom is easily detectable and it is easy to measure the two layers, to determine the percentage of red blood cells. Modern cell analyzers check a number of blood factors at once, and obtain the hematocrit indirectly from the number and size of the red blood cells.

Normal hematocrit test results range from 38-46% for an adult female, and 42-54% for an adult male. A result of 50% indicates that red blood cells make up half of the blood. Hematocrit testing is frequently used to screen for anemia, a deficiency of red blood cells, or to monitor the treatment of an existing anemic condition. If there is an inadequate number of red blood cells, the blood will not transport enough oxygen through the body, and the person will often feel fatigued.

A low hematocrit blood test can also indicate hemorrhaging, which can be distinguished from chronic blood loss by an examination of the size of the red blood cells and the size of the red cell population. Other reasons for low test results can include the destruction of red blood cells, bone marrow failure, leukemia, Vitamin B12 and folic acid deficiencies, some cancers, and pregnancy.

The hematocrit test is one of the factors used in deciding whether or not to give a blood transfusion. If the value is below 21%, a transfusion is merited. This test is also valuable to estimate the number of transfusions needed. For each unit of red blood cells given to an adult, the hematocrit should increase by 3-4%.

Hematocrit test results that are elevated can also be problematic. If there are too many red blood cells, blood may not flow easily through the small capillaries. There is a disease called polycythemia vera, in which the person has an above average number of red blood cells in their bone marrow. This may be due to a deficit in lung function, such as chronic obstructive pulmonary disease (COPD).

A common reason for high levels is dehydration. Repeating the test after re-hydration will sometimes give normal levels. Other reasons for having a high hematocrit from dehydration include burns and diarrhea.

Athletes are given hematocrit tests to see if they have taken erythropoietin as a performance-enhancement drug. Erythropoietin is a hormone produced by the kidneys that stimulates the bone marrow to produce red blood cells. By having more red blood cells, the athletes can transport more oxygen in their blood and be better able to compete. To check whether performance-enhancement drugs may be in use, athletes' long-term hematocrit levels are compared with a general, absolute-permitted maximum level.