The hematocrit is a test that measures the percentage of blood that is comprised of red blood cells.
The hematocrit is used to screen for anemia, or is measured on a person to determine the extent of anemia. An anemic person has fewer or smaller than normal red blood cells. A low hematocrit, combined with other abnormal blood tests, confirms the diagnosis. The hematocrit is decreased in a variety of common conditions including chronic and recent acute blood loss, some cancers, kidney and liver diseases, malnutrition, vitamin B 12 and folic acid deficiencies, iron deficiency, pregnancy, systemic lupus erythematosus, rheumatoid arthritis and peptic ulcer disease. An elevated hematocrit is most often associated with severe burns, diarrhea, shock, Ad dison's disease, and dehydration, which is a decreased amount of water in the tissues. These conditions reduce the volume of plasma water causing a relative increase in RBCs, which concentrates the RBCs, called hemoconcentration. An elevated hematocrit may also be caused by an absolute increase in blood cells, called polycythemia. This may be secondary to a decreased amount of oxygen, called hypoxia, or the result of a proliferation of blood forming cells in the bone marrow (polycythemia vera).
Critically high or low levels should be immediately called to the attention of the patient's nurse or doctor. Transfusion decisions are based on the results of laboratory tests, including the hematocrit. Generally, transfusion is not considered necessary if the hematocrit is above 21%. The hematocrit is also used as a guide to how many transfusions are needed. Each unit of packed red blood cells administered to an adult is expected to increase the hematocrit by approximately 3% to 4%.
Fluid volume in the blood affects hematocrit values. Accordingly, the blood sample should not be taken from an arm receiving IV fluid or during hemodialysis. It should be noted that pregnant women have extra fluid, which dilutes the blood, decreasing the hematocrit. Dehydration concentrates the blood, which increases the hematocrit.
In addition, certain drugs such as penicillin and chloramphenicol may decrease the hematocrit, while glucose levels above 400 mg/dL are known to elevate results. Blood for hematocrit may be collected either by finger puncture, or sticking a needle into a vein, called venipuncture. When performing a finger puncture, the first drop of blood should be wiped away because it dilutes the sample with tissue fluid. A nurse or phlebotomist usually collects the sample following cleaning and disinfecting the skin at the site of the needle stick.
Blood is made up of red blood cells, white blood cells (WBCs), platelets, and plasma. A decrease in the number or size of red cells also decreases the amount of space they occupy, resulting in a lower hematocrit. Conversely, an increase in the number or size of red cells increases the amount of space they occupy, resulting in a higher hematocrit. Thalassemia minor is an exception in that it usually causes an increase in the number of red blood cells, but because they are small, it results in a decreased hematocrit.
The hematocrit may be measured manually by centrifugation. A thin capillary tube called a microhematocrit tube is filled with blood and sealed at the bottom. The tube is centrifuged at 10,000 RPM (revolutions per minute) for five minutes. The RBCs have the greatest weight and are forced to the bottom of the tube. The WBCs and platelets form a thin layer, called the buffy coat, between the RBCs and the plasma, and the liquid plasma rises to the top. The height of the red cell column is measured as a percent of the total blood column. The higher the column of red cells, the higher the hematocrit. Most commonly, the hematocrit is measured indirectly by an automated blood cell counter. It is important to recognize that different results may be obtained when different measurement principles are used. For example, the microhematocrit tube method will give slightly higher results than the electronic methods when RBCs of abnormal shape are present because more plasma is trapped between the cells.
Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn, and lying down and relaxing for awhile is helpful for these people.
Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.
Normal values vary with age and sex. Some representative ranges are:
Chernecky, Cynthia C. and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia: W. B. Saunders Company, 2001.
Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.
Kjeldsberg, Carl R. Practical Diagnosis of Hematologic Disorders. 3rd ed. Chicago: ASCP Press, 2000.
American Association of Blood Banks. 8101 Glenbrook Road, Bethesda, Maryland 20814. (301) 907-6977. Fax: (301) 907-6895. http://www.aabb.org .
Uthman, Ed. Blood Cells and the CBC. 2000 [cited February 17, 2003]. http://web2.iadfw.net/uthman/blood_cells.html .
Victoria E. DeMoranville
Mark A. Best