Hematocrit Test

Why this test may be ordered

• As part of a routine checkup
• To monitor health if you are being treated for cancer or have an ongoing health condition
• If you have symptoms of anemia, such as shortness of breath, weakness, fatigue, headache, dizziness, or arrhythmia
• If you have symptoms of polycythemia, such as headache, feeling light-headed, shortness of breath when lying down, weakness, skin itching after a shower, blurred vision, or unusual bleeding

Preparation and collection

You don't need any special preparations for a hematocrit test. If your provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. This usually takes less than five minutes.

There is very little risk to a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

Understanding results

Your hematocrit test results are reported as a number, which is the percentage of your blood that's made up of red blood cells. For example, if your hematocrit test result is 42, it means that 42% of your blood is red blood cells, and the rest is made up of white blood cells, platelets, and blood plasma. Normal hematocrit levels vary depending on your sex, age, whether or not you smoke, and the altitude where you live. Ask your provider what your hematocrit level should be.

• Lower than normal: May indicate blood loss from a recent injury, chronic blood loss, or your body not having enough red blood cells (anemia). It may also be caused by your body making too many white blood cells due to bone marrow disease or certain cancers (such as leukemia, lymphoma, multiple myeloma, or cancers that spread to the bone marrow). Limitation: A low hematocrit can also occur normally during pregnancy because your body has more fluid than usual, which decreases the percentage of red blood cells. A low result does not always mean you have a medical condition that needs treatment. Next step: Talk with your provider to learn what your test results mean.
• Higher than normal: May indicate your body is making too many red blood cells (polycythemia) or that your blood plasma level is too low (dehydration or shock). It can also be caused by lung disease, congenital heart disease, or heart failure. Limitation: A high hematocrit can also occur normally if you live at high altitudes with less oxygen in the air, because your body responds by making more red blood cells. A high result does not always mean you have a medical condition that needs treatment. Next step: Talk with your provider to learn what your test results mean.

• Normal hematocrit levels vary depending on your sex, age, whether or not you smoke, and the altitude where you live.
• Pregnancy can cause a low hematocrit because your body has more fluid than usual, which decreases the percentage of red blood cells.
• Living at high altitudes with less oxygen in the air may cause a high hematocrit.
• If your results are not in the normal range, it doesn't always mean you have a medical condition that needs treatment.
• Interpretation of results should be done by your healthcare provider.

What to do next

To learn what your test results mean, talk with your provider.

• What is my hematocrit level and what does it mean for my health?
• Do I need any follow-up tests based on my hematocrit result?
• Could my medications or other health conditions affect my hematocrit level?
• How often should I have my hematocrit checked?
• What symptoms should I watch for that might indicate a change in my red blood cell levels?

Evidence source

U.S. National Library of Medicine: Hematocrit Test