Hematocrit is a blood test that measures the percentage of red blood cells found in whole blood. This measurement depends on the number of red blood cells and the size of red blood cells.

The hematocrit is almost always ordered as part of a complete blood count.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any 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%

Normal value ranges vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

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

High hematocrit may be due to:

• Dehydration
  • Burns
  • Diarrhea
• Erythrocytosis
• Low blood oxygen levels (hypoxia)
• Polycythemia vera

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood 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)

Zuckerman K. Approach to the anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 162.