Hematocrit is a blood test that measures the percentage of the volume of whole blood that is made up of red blood cells. This measurement depends on the number of red blood cells and the size of red blood cells.
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
No special preparation is necessary for this test.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
The hematocrit is almost always ordered as part of a complete blood count (CBC).
Your doctor may order this test if you have signs of:
- Diet deficiency
- Other medical condition
Normal results vary, but in general are as follows:
- Male: 40.7 to 50.3%
- Female: 36.1 to 44.3%
Normal results for children vary, but in general are:
- Newborn: 45 to 61%
- Infant: 32 to 42%
The examples above are common measurements for results of these tests. Normal value ranges vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Low hematocrit may be due to:
- Destruction of red blood cells
- Nutritional deficiencies of iron, folate, vitamin B12, and vitamin B6
High hematocrit may be due to:
- Congenital heart disease
- Failure of the right side of the heart (cor pulmonale)
- Abnormal increase in red blood cells (erythrocytosis)
- Low blood oxygen levels (hypoxia)
- Scarring or thickening of the lungs (pulmonary fibrosis)
- Bone marrow disease that causes abnormal increase in RBCs (polycythemia vera)
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Hutchison RE, McPherson RA, Schexneider KI. Basic examination of blood and bone marrow. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 30.
Reviewed By: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.