A hematocrit test measures the proportion of red blood cells in the blood, which can indicate anemia or polycythemia vera. It is used to monitor treatment and decide on blood transfusions. Athletes are tested to detect performance-enhancing drugs.
A hematocrit test measures the proportion of your blood volume 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).
Centrifugation takes place in fine capillary tubes. The blood separates into two layers. The red blood cell layer at the bottom is easily detected, and it is easy to measure the two layers, to determine the percentage of red blood cells. Modern cell analyzers monitor a number of blood factors simultaneously and obtain hematocrit indirectly from the number and size of 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. The hematocrit test is often used to screen for anemia, a shortage of red blood cells, or to monitor treatment for an existing anemic condition. If there is an inadequate number of red blood cells, the blood will not carry enough oxygen through the body and the person will often feel fatigued.
A low hematocrit blood test may also indicate hemorrhage, which can be distinguished from chronic blood loss by an examination of red blood cell size and red blood cell population. Other reasons for low test results can include destruction of red blood cells, bone marrow failure, leukemia, vitamin B12 and folic acid deficiencies, certain cancers, and pregnancy.
The hematocrit test is one of the factors used to decide whether or not to give a blood transfusion. If the value is less than 21%, a transfusion is deserved. This test is also useful for estimating the number of transfusions needed. For every unit of red blood cells given to an adult, the hematocrit should increase by 3-4%.
Elevated hematocrit test results can also be problematic. If there are too many red blood cells, blood may not flow easily through the tiny capillaries. There is a disease called polycythemia vera, in which the person has an above-average number of red blood cells in the 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. Retesting after rehydration will sometimes give normal levels. Other reasons for having an elevated hematocrit from dehydration include burns and diarrhea.
Athletes are given hematocrit tests to see if they have been taking erythropoietin as a performance-enhancing 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, athletes can carry more oxygen in their blood and be better able to compete. To test whether performance-enhancing drugs may be in use, athletes’ long-term hematocrit levels are compared to a general, absolute maximum level allowed.
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