Mayo Clinic Health Library

Hematocrit test

Updated: 07-10-2013

Definition

Hematocrit (he-MAT-uh-krit) is the proportion of your total blood volume that is composed of red blood cells. A hematocrit (Hct) test indicates whether you have too few or too many red blood cells — conditions that can occur as the result of certain diseases. Red blood cells, or erythrocytes (uh-RITH-roe-sites), transport oxygen throughout your body.

A hematocrit test is done using a sample of your blood. A lab technician puts the sample in a device called a centrifuge that spins the blood very quickly in a test tube. This motion separates your blood into three parts: the fluid component (plasma), red blood cells and other blood cells. When the blood is separated, the technician can determine what proportion of the cells are red blood cells. Hematocrit is also called packed-cell volume (PCV).

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Why it's done

A hematocrit test is part of a complete blood count (CBC). The proportion of red blood cells compared with all blood cells may help your doctor make a diagnosis or monitor your response to a treatment.

A lower than normal hematocrit may indicate:

  • An insufficient supply of healthy red blood cells (anemia)
  • A large number of white blood cells — usually a very small portion of your blood — due to long-term illness, infection, leukemia, lymphoma or other disorders of white blood cells
  • Vitamin or mineral deficiencies
  • Recent or long-term blood loss

A higher than normal hematocrit may indicate:

  • Dehydration
  • A disorder, such as polycythemia vera, that causes your body to produce too many red blood cells
  • Lung or heart disease — if the body senses low oxygen levels, it will make more red blood cells in an effort to increase the amount of oxygen in the blood
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How you prepare

The hematocrit is a simple blood test. You won't need to fast before the test or make other preparations.

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What you can expect

A nurse or medical assistant may collect a blood sample by pricking your finger and collecting the blood into a narrow glass tube or by drawing blood with a needle from a vein in your arm. You may feel some tenderness at the site, but you'll be able to resume normal activities after the sample is drawn.

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Results

Results from your hematocrit are reported as the percentage of blood cells that are red blood cells. The normal range is 38.8 to 50 percent for men and 34.9 to 44.5 percent for women. The normal range for children 15 years of age and younger varies by age and sex. The lower and upper thresholds for a normal hematocrit value may vary somewhat from one medical practice to another.

The result of your hematocrit test is just one piece of information that helps your doctor check your health. Talk to your doctor about what your hematocrit test result means in light of the symptoms you're experiencing and the results of other diagnostic tests.

Accuracy of test results
A number of conditions can affect the outcome of a hematocrit test and yield inaccurate or misleading results. These complicating factors include:

  • Living at a high altitude
  • Pregnancy
  • Significant recent blood loss
  • Recent blood transfusion
  • Severe dehydration

Your doctor will take into account possible complicating factors when interpreting the results of your hematocrit test. Your doctor may want to repeat the hematocrit test and do other blood tests if results provide conflicting or unexpected information.

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