Hematocrit Test

What is a hematocrit test?

A hematocrit test is a blood test that measures the amount (percent) of your blood that is made up of red blood cells. Red blood cells carry oxygen from your lungs to the rest of your body. The other parts of your blood include white blood cells (to help fight infection and other diseases), platelets (to help make blood clots to stop bleeding), and a liquid called plasma.

Hematocrit levels that are too high or too low can be a sign of a blood disorder, dehydration, or other medical conditions that affect your blood.

Other names: HCT, packed cell volume, PCV, Crit; H and H (Hemoglobin and Hematocrit)

What is it used for?

A hematocrit (HCT) test is often part of a complete blood count (CBC). A CBC is a common blood test that measures the number and type of cells in your blood. It is used to check your general health. It may also be used to monitor or help diagnose blood disorders. These could include anemia, a condition in which you don't have enough red blood cells, and polycythemia (erythrocytosis), an uncommon disorder in which you have too many red blood cells and your blood becomes too thick.

Why do I need a hematocrit test?

Your health care provider may order a hematocrit test as part of your routine checkup or to monitor your health if you are being treated for cancer or have an ongoing health condition. Your provider may also order this test if you have symptoms of a red blood cell disorder, such as anemia or polycythemia:

Symptoms of anemia (too few red blood cells) may include:

• Shortness of breath
• Weakness or fatigue
• Headache
• Dizziness
• Arrhythmia (a problem with the rate or rhythm of your heartbeat)

Symptoms of polycythemia (too many red blood cells) may include:

• Headache
• Feeling light-headed or dizzy
• Shortness of breath when lying down
• Weakness or fatigue
• Skin symptoms such as itching after a shower or bath, burning, or a red face
• Heavy sweating, especially during sleep
• Blurred or double vision and blind spots
• Unusual bleeding, such as bleeding gums or nosebleeds

What happens during a hematocrit test?

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. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

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.

Are there any risks to the test?

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.

What do the results mean?

Your hematocrit test results are reported as a number. That number 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 will 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.

A lower-than-normal hematocrit level may be a sign that:

• You have had blood loss due to a recent injury
• You have a condition causing chronic (long-term) blood loss
• Your body doesn't have enough red blood cells (anemia). There are many types of anemia, and they can be caused by different medical conditions.
• Your body is making too many white blood cells, which may be caused by:
  • Bone marrow disease
  • Certain cancers, including leukemia, lymphoma, multiple myeloma, or cancers that spread to your bone marrow from other parts of the body

A higher-than-normal hematocrit level may be a sign that:

• Your body is making too many red blood cells, which may be caused by:
  • Lung disease
  • Congenital heart disease
  • Heart failure
  • Polycythemia
• Your blood plasma level is too low, which may be caused by:
  • Dehydration, the most common cause of a high hematocrit
  • Shock

If your results are not in the normal range, it doesn't always mean you have a medical condition that needs treatment. Living at high altitudes with less oxygen in the air may cause a high hematocrit. That's because your body responds to low oxygen levels by making more red blood cells to give you the oxygen you need.

Pregnancy can cause a low hematocrit. That's because your body has more fluid than usual during pregnancy, which decreases the percentage that's made of red blood cells.

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

Learn more about laboratory tests, reference ranges, and understanding results.