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Hematocrit

Reviewed on 11/10/2020
A nurse puts a blood sample into an automated machine for hematocrit measurements.
Source: Getty Images

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, a 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 of the blood 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 then be visually measured.

A woman has her blood drawn.
Source: Getty Images

What is a normal hematocrit ranges by ages?

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

AgeHematocrit Ranges
Newborn55% to 68%
One (1) week of age47% to 65%
One (1) month of age37% to 49%
Three (3) months of age30% to 36%
One (1) year of age29% to 41%
Ten (10) years of age36% to 40%
Adult males42% to 54%
Adult women38% to 46%

These values may vary slightly among different laboratories.

A stethoscope and the word anemia.
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What does a low hematocrit mean?

A person who has a low hematocrit is referred to as being anemic. There are many reasons for anemia. Some of the more common reasons are

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A man getting IV fluids.
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What does a high hematocrit mean?

Higher than normal hematocrit levels represent abnormally elevated red blood cell counts.

  • High hematocrits can be seen in people living at high altitudes and in chronic smokers.
  • Dehydration produces a falsely high hematocrit that disappears when the proper fluid balance is restored.

Some other infrequent causes of an elevated hematocrit are

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References
Tintinalli J, et al. Tintinalli's Emergency Medicine: A Comprehensive Study Guide, Eighth Edition. Mcgraw-Hill Education/Medical, 2015.
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