This test is used to evaluate:

• anemia,
• polycythemia,
• response to treatment of anemia or polycythemias,
• dehydration,
• blood transfusion decisions for severe symptomatic anemias, and
• the effectiveness of those transfusions.

The hematocrit is normally ordered as a part of the complete blood count (CBC). It is also repeated at regular intervals for many conditions, including:

• the diagnosis of anemia and polycythemia,
• the monitoring of treatment for anemia,
• recovery from dehydration, and
• monitoring of ongoing bleeding to check its severity.

Decreased hematocrit indicates anemia, such as that caused by iron deficiency or other deficiencies. Further testing may be necessary to determine the exact cause of the anemia.

Other conditions that can result in a low hematocrit include vitamin or mineral deficiencies, recent bleeding, cirrhosis of the liver, and malignancies.

The most common cause of increased hematocrit is dehydration, and with adequate fluid intake, the hematocrit returns to normal. However, it may reflect a condition called polycythemia vera—that is, when a person has more than the normal number of red blood cells. This can be due to a problem with the bone marrow or, more commonly, as compensation for inadequate lung function (the bone marrow manufactures more red blood cells in order to carry enough oxygen throughout your body). Anytime a hematocrit is persistently high, the cause should be determined in consultation with a doctor.

With regard to transfusions, this is normally not considered for otherwise healthy persons as long as the hemoglobin level is above 8 grams per deciliter or the hematocrit is above 24%.

Pregnancy usually causes slightly decreased hematocrit values due to extra fluid in the blood.

Living at high altitudes causes increased hematocrit values—this is your body’s response to the decreased oxygen available at these heights.