Hematocrit
At a Glance
Why Get Tested?
If your doctor suspects that you have anemia, polycythemia, or dehydration
When to Get Tested?
As part of a complete blood count (CBC), which may be ordered for a variety of reasons
Sample Required?
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)
Test Preparation Needed?
None
The Test Sample
What is being tested?
Hematocrit is a measurement of the proportion of blood that is made up of red blood cells. The value is expressed as a percentage or fraction of cells in blood. For example, a hematocrit value of 40% means that there are 40 milliliters of red blood cells in 100 milliliters of blood.
The hematocrit rises when the number of red blood cells increases or when the plasma volume is reduced, as in dehydration. The hematocrit falls to less than normal, indicating anemia, when your body decreases its production of red blood cells or increases its destruction of red blood cells or if blood is lost due to bleeding.
The hematocrit reflects both the number of red cells and their volume (mean corpuscular volume or MCV). If the size of the red cell decreases, so will the hematocrit and vice versa.
How is the sample collected for testing?
A sample is obtained by drawing blood through a needle placed in a vein in the arm or by a fingerstick (for children and adults) or a heelstick (for newborns).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
This test is used to evaluate:
- anemia,
- polycythemia,
- response to treatment of anemia or polycythemias,
- dehydration,
- blood transfusion decisions for severe symptomatic anemias, and
- the effectiveness of those transfusions.
When is it ordered?
The hematocrit is normally ordered as a part of the complete blood count (CBC). It is also repeated at regular intervals for many conditions, including:
- the diagnosis of anemia and polycythemia,
- the monitoring of treatment for anemia,
- recovery from dehydration, and
- monitoring of ongoing bleeding to check its severity.
What does the test result mean?
Decreased hematocrit indicates anemia, such as that caused by iron deficiency or other deficiencies. Further testing may be necessary to determine the exact cause of the anemia.
Other conditions that can result in a low hematocrit include vitamin or mineral deficiencies, recent bleeding, cirrhosis of the liver, and malignancies.
The most common cause of increased hematocrit is dehydration, and with adequate fluid intake, the hematocrit returns to normal. However, it may reflect a condition called polycythemia verathat is, when a person has more than the normal number of red blood cells. This can be due to a problem with the bone marrow or, more commonly, as compensation for inadequate lung function (the bone marrow manufactures more red blood cells in order to carry enough oxygen throughout your body). Anytime a hematocrit is persistently high, the cause should be determined in consultation with a doctor.
With regard to transfusions, this is normally not considered for otherwise healthy persons as long as the hemoglobin level is above 8 grams per deciliter or the hematocrit is above 24%.
Is there anything else I should know?
Pregnancy usually causes slightly decreased hematocrit values due to extra fluid in the blood.
Living at high altitudes causes increased hematocrit valuesthis is your bodys response to the decreased oxygen available at these heights.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
S1
Thomas, Clayton L., Editor (1997). Tabers Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
S2
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosbys Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
S3
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 514-517.
