Hematocrit

Medically reviewed by: William C. Shiel, Jr., MD, FACP, FACR

• What is the hematocrit?
• How is the hematocrit measured?
• What is a normal hematocrit?
• What does a low hematocrit mean?
• What does a high hematocrit mean?

What is the hematocrit?

The hematocrit is the proportion, by volume, of the blood that consists of red blood cells. The hematocrit (hct) is expressed as a percentage. For example, an hematocrit of 25% means that there are 25 milliliters of red blood cells in 100 milliliters of blood.

How is the hematocrit measured?

The hematocrit is typically measured from a blood sample by an automated machine that makes several other measurements at the same time. Most of these machines in fact do not directly measure the hematocrit, but instead calculate it based on the determination of the amount of hemoglobin and the average volume of the red blood cells. The hematocrit can also be determined by a manual method using a centrifuge. When a tube of blood is centrifuged, the red cells will be packed into the bottom of the tube. The proportion of red cells to the total blood volume can be visually measured.

What is a normal hematocrit?

The normal ranges for hematocrit are dependent on age and, after adolescence, the sex of the individual. The normal ranges are:

• Newborns: 55%-68%
  
  
• One (1) week of age: 47%-65%
  
  
• One (1) month of age: 37%-49%
  
  
• Three (3) months of age: 30%-36%
  
  
• One (1) year of age: 29%-41%
  
  
• Ten (10) years of age: 36%-40%
  
  
• Adult males: 42%-54%
  
  
• Adult women: 38%-46%

These values may vary slightly between laboratories.

What does a low hematocrit mean?

A low hematocrit is referred to as being anemic. There are many reasons for anemia. Some of the more common reasons are loss of blood (traumatic injury, surgery, bleeding colon cancer), nutritional deficiency (iron, vitamin B12, folate), bone marrow problems (replacement of bone marrow by cancer, suppression by chemotherapy drugs, kidney failure), and abnormal hematocrit (sickle cell anemia).

What does a high hematocrit mean?

Higher than normal hematocrit levels can be seen in people living at high altitudes and in chronic smokers. Dehydration produces a falsely high hematocrit that disappears when proper fluid balance is restored. Some other infrequent causes of elevated hematocrit are lung disease, certain tumors, a disorder of the bone marrow known as polycythemia rubra vera, and abuse of the drug erythropoietin (Epogen) by athletes for blood doping purposes.

Reference:
Harrison's Principles of Internal Medicine, McGraw-Hill, edited by Eugene Braunwald, et. al., 2001.

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