Hematocrit Level Test

Low, High, and Normal Ranges

In This Article

A hematocrit (HCT) test measures the number of red blood cells (RBCs) you have in relation to white blood cells and platelets. Also known as a packed-cell volume (PCV) test, the HCT is a simple blood test usually done along with other tests to help doctors determine whether you have a blood or bone marrow disorder, nutritional deficiency, or other condition. Often, a hemoglobin (Hgb) test is done with an HCT blood test (“crit” for short); the pair of tests is called an H and H test.

What Is Hematocrit?

Hematocrit is the percentage, by volume, of how much of your blood contains red blood cells (RBCs). Measuring this percentage reveals whether you have too many or too few red blood cells, which can be used to diagnose certain diseases.

Purpose of Test 

By determining what percentage of your blood consists of red blood cells, an HCT test can be an early indicator of whether you have a condition related to too few or too many RBCs.

Commonly, doctors use the test to check for anemia, a blood disorder related to a low RBC count that causes fatigue, headaches, and dizziness. The test also may be used to screen for polycythemia vera (PV), a rare blood disease that enlarges the spleen and also causes fatigue and headaches.

If you're undergoing cancer treatment, an HCT will be one of several routine tests used to check how you're responding to medication and to enable doctors to manage side effects.

Procedure

As a basic blood test, the HCT doesn’t require fasting or special considerations. Blood is drawn from the elbow or the back of the hand. With infants and young children, the phlebotomist may make a small puncture in the heel or fingertip and collect blood onto a test strip.

An HCT test is usually ordered as part of a complete blood count (CBC) in which multiple tests are run from blood taken at one time, so you may need to have more than one vial of blood drawn.

In addition to the full CBC, other tests may be performed at the same time or as a follow-up to help with a diagnosis, including:

  • A blood smear: A small blood sample of blood is examined under a microscope for signs of problems.
  • Reticulocyte count: A blood test that determines how well your bone marrow makes RBCs.
  • Iron studies: A group of blood tests interpreted altogether to check for iron deficiency or iron overload.
  • Vitamin B12 and folate levels: A test to see if you have enough of the nutrients needed to create blood cells.

How Blood Is Tested

Once your blood is collected, it’s placed in a centrifuge or an automated hematology analyzer instrument. The centrifuge separates the erythrocytes (red blood cells) from the white blood cells and platelets, giving a direct measure of the percentage of RBCs in the blood sample.

The automated instrument, which calculates the average RBC volume rather than a direct measurement, provides quick results, but can produce false positive results. Depending on the purpose of the test, your doctor will determine which method is best.

Interpreting Results

Many factors affect hematocrit level, but general guidelines for normal levels in adults and young children are:

  • Newborns (0 to 3 days): 45% to 67%
  • Newborns (3 days to 1 week): 42% to 66%
  • Infants (1 to 2 weeks): 39% to 63%
  • Infants (2 weeks to 1 month): 31% to 55%
  • Infants (1 to 2 months): 28% to 42%
  • Infants (2 months to 6 months): 29% to 41%
  • Babies (6 months to 2 years): 33% to 39%
  • Children (2 to 6 years): 34% to 40%
  • Children (6 to 12 years): 35% to 45%
  • Females (12 years to adult): 36% to 46%
  • Males (12 to 18 years): 37% to 49%
  • Males (18 years to adult): 41% to 53%

If your levels are below or above normal, you could be at risk for one of these serious health conditions.

Hematocrit Levels Outside Normal Range
Below Normal
too few RBCs results in anemia,
which may be caused by: 
Above Normal
too many RBCs, which
may be caused by:
Blood loss due to internal or external trauma Heart disease 
Leukemia, lymphoma, or other bone marrow cancers
 
Dehydration
Iron or vitamin deficiency including folate, B12, or B6
 

Scarring/thickening of the lungs

Too much water in the body

PV/other bone marrow disease 

Kidney disease

Obstructive sleep apnea

Thyroid problems

Smoking

Bone marrow damage from chemotherapy or toxins

Carbon monoxide poisoning

Autoimmune disease such as Evans syndrome 

Elevated testosterone levels 

HCT test results may be affected by other factors such as a recent blood transfusion, pregnancy, or living at a high altitude.

Follow-Up

Your doctor should review with you the results of the HCT and all other tests that were done. If the results indicate that you're anemic, you can take some simple steps to improve your red blood cell count by fortifying your diet or using over-the-counter iron supplements. If anemia is the result of an infection, you may need to begin antibiotics.

When test results point to a bone marrow problem, a bone marrow examination may be needed in which a sample of bone marrow is taken, usually from the hip, to check for lymphoma and similar disorders.

Conditions related to an overproduction of RBCs may require lifestyle changes such as quitting smoking or adopting a heart-healthy diet. In the rare case of PV, doctors can provide medication that will help you manage the condition, but it cannot be cured.

Anytime your levels are outside the normal hematocrit range, your doctor will likely repeat the HCT test at least annually to check on your condition.

A Word From Verywell

No one likes to get pricked with a needle. However, an HCT test is one of those "worth the pain" tests. It's quick and easy and goes a long way in helping doctors get a more complete picture of your overall health. The source of everyday problems such as fatigue or headaches might be revealed as simple anemia or a more serious blood disorder that needs immediate care. In either instance, one simple blood draw can make a huge difference. 

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Article Sources
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