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Hematocrit

Definition

Hematocrit is a blood test that measures how much of a person's blood is made up of red blood cells as opposed to plasma. This measurement depends on the number of and size of the red blood cells.

How the Test is Performed

A blood sample is needed.

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. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Why the Test is Performed

The hematocrit is almost always done as part of a complete blood count (CBC).

Your health care provider may recommend this test if you have signs of or are at risk for anemia. These include having:

• Before and after major surgery

• Blood in your stools, or vomit (if you throw up)

• Chronic medical problems, such as kidney disease or certain types of arthritis

• During pregnancy

• Fatigue, poor health, or unexplained weight loss

• Headaches

• Heavy menstrual periods

• Leukemia or other problems in the bone marrow

• Monitoring during treatment for cancer

• Monitoring medicines that may cause anemia or low blood counts

• Monitoring of anemia and its cause

• Poor nutrition

• Problems concentrating

Normal Results

Normal results vary, but in general they are:

• Male: 40.7% to 50.3%

• Female: 36.1% to 44.3%

For babies, normal results are:

• Newborn: 45% to 61%

• Infant: 32% to 42%

The examples above are common measurements for results of these tests. Normal value ranges vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

Low hematocrit may be due to:

• Anemia

• Bleeding

• Bone marrow being unable to produce new red blood cells. This may be due to leukemia, other cancers, medicine toxicity, radiation therapy, infection, or bone marrow disorders

• Chronic illness

• Chronic kidney disease

• Destruction of red blood cells (hemolysis)

• Leukemia

• Malnutrition

• Pregnancy

• Too little iron, folate, vitamin B12, and vitamin B6 in the diet

• Too much water in the body

High hematocrit may be due to:

• Bone marrow disease that causes abnormal increase in RBCs (polycythemia vera)

• Cigarette smoking

• Dehydration (for example, from severe diarrhea)

• Failure of the right side of the heart (cor pulmonale)

• Kidney tumor (renal cell carcinoma)

• Low blood oxygen level (hypoxia)

• Problem with heart's structure and function that is present at birth (congenital heart disease)

• Scarring or thickening of the lungs (pulmonary fibrosis)

Your hematocrit will increase for several weeks when you are in a higher altitude.

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person 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 lightheaded

• Multiple punctures to locate veins

• Hematoma (blood buildup under the skin)

• Infection (a slight risk any time the skin is broken)

References

Means RT Jr. Approach to the anemias. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 144.

Thom CS, Lambert MP. Blood disorders. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 138.

Vajpayee N, Graham SS, Bem S. Basic examination of blood and bone marrow. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 31.

Review Date: 3/11/2024

Reviewed By: Frank D. Brodkey, MD, FCCM, Associate Professor, Section of Pulmonary and Critical Care Medicine, University of Wisconsin School of Medicine and Public Health, Madison, WI. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.