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Hematocrit

Definition

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.

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

Alternative Names

HCT

How the test is performed

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.

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, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Your doctor may order this test if you have signs of:

• Anemia
• Diet deficiency
• Leukemia
• Other medical condition

Normal Values

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.

What abnormal results mean

Low hematocrit may be due to:

• Anemia
• Bleeding
• Destruction of red blood cells
• Leukemia
• Malnutrition
• Nutritional deficiencies of iron, folate, vitamin B12, and vitamin B6
• Overhydration

High hematocrit may be due to:

• Congenital heart disease
• Cor pulmonale
• Dehydration
• Erythrocytosis
• Low blood oxygen levels (hypoxia)
• Pulmonary fibrosis
• Polycythemia vera

What the risks are

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)

References

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