Hematocrit is a blood test that measures the number of red blood cells and the size of red blood cells. It gives a percentage of red blood cells found in whole blood. This test is almost always ordered as part of a complete blood count.

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

No special preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Your doctor may order this test if you have signs of anemia, leukemia, diet deficiency, or other medical condition.

Normal results vary, but in general are as follows:

• Male: 40.7 - 50.3%
• Female: 36.1 - 44.3%

Low hematocrit may be due to:

• Anemia
• Blood loss (hemorrhage)
• Bone marrow failure
• Destruction of red blood cells
• Leukemia
• Malnutrition or specific diet deficiencies
• Multiple myeloma
• Rheumatoid arthritis

• Dehydration
  • Burns
  • Diarrhea
• Erythrocytosis
• Polycythemia vera

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

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.

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:459-60.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2674.