A hematocrit (he-MAT-uh-krit) test measures the proportion of red blood cells in the blood. Red blood cells carry oxygen throughout the body. Having too few or too many of them can be a sign of certain diseases.

The hematocrit test is a simple blood test. It is sometimes referred to as a packed-cell volume test.

A hematocrit test can help your health care team make a diagnosis or monitor how you respond to a treatment. The test is done as part of a complete blood count (CBC).

When the hematocrit value is low, the proportion of red blood cells in the blood is lower than usual. This can indicate:

• The blood has too few healthy red blood cells. This condition is called anemia.
• That the body does not have enough vitamins or minerals.
• Recent or long-term blood loss.

When the hematocrit value is high, the proportion of red blood cells in the blood is higher than normal. This can indicate:

• Dehydration.
• A disorder that causes your body to produce too many red blood cells, such as polycythemia vera.
• Lung or heart disease.
• Living at a high altitude, such as on a mountain.

The hematocrit is a simple blood test. You won't need to fast before the test or make other preparations.

The blood sample is generally drawn with a needle from a vein in your arm. You may feel some tenderness at the site, but you'll be able to resume normal activities afterward.

Results from your hematocrit test are reported as the percentage of blood cells that are red blood cells. Typical ranges vary substantially with race, age and sex. The definition of typical red-blood cell percentage also may vary somewhat from one medical practice to another. This is because laboratories decide what is a healthy range based on the population in their area.

Generally, a typical range is considered to be:

• For men, 38.3% to 48.6%.
• For women, 35.5% to 44.9%.

For children ages 15 and younger, the typical range varies by age and sex.

Your hematocrit test provides just one piece of information about your health. Talk to your health care team about what your hematocrit test result means.

Accuracy of test results

A number of factors can affect the outcome of a hematocrit test. There are some situations when hematocrit is not in the typical range but it does not mean a person is sick. These include:

• Living at a high altitude increases hematocrit.
• Pregnancy lowers hematocrit.
• Recent loss of a lot of blood lowers hematocrit.
• Recent blood transfusion may raise hematocrit.
• Severe dehydration may raise hematocrit.

Your health care team will consider possible complicating factors when interpreting the results of your hematocrit test. If results provide conflicting or unexpected information, they may want to repeat the hematocrit test and do other blood tests.