Hematocrit is the percentage of red blood cells in the total blood volume. Red blood cells are vital to your health. Imagine them as the subway system of your blood. They transport oxygen and nutrients to various locations in your body. For you to stay healthy, your body needs to have the correct proportion of red blood cells.
Your doctor may order a hematocrit, or Hct, test if they think you have too few or too many red blood cells.
A hematocrit test can help your doctor diagnose you with a particular condition, or it can help them determine how well your body is responding to a certain treatment. The test can be ordered for a variety of reasons, but it’s most often used to test for:
If your doctor orders a complete blood count (CBC) test, the hematocrit test is included. Other tests in a CBC are a hemoglobin and reticulocyte count. Your doctor will look at your overall blood test results to gain an understanding of your red blood cell count.
First you will receive a blood test. Afterward, it will be sent to a laboratory for evaluation.
A medical provider will need a small sample of blood to test your hematocrit. This blood can be drawn from a finger prick or taken from a vein in your arm.
If the hematocrit test is part of a CBC, a lab technician will draw blood from a vein, typically from the inside of your elbow or from the back of your hand. The technician will clean the surface of your skin with an antiseptic and place an elastic band, or tourniquet, around your upper arm to help the vein swell with blood.
They’ll then insert a needle in the vein and collect a blood sample in one or more vials. The technician will remove the elastic band and cover the area with a bandage to stop the bleeding. A blood test can be slightly uncomfortable. When the needle punctures your skin, you might feel a prick or pinching sensation. Some people also feel faint or lightheaded when they see blood. You may experience minor bruising, but this will clear up within a few days. The test will take only a few minutes, and you can resume everyday activities after it’s finished. Your sample will be sent to a lab for analysis.
In the laboratory, your hematocrit is evaluated using a centrifuge, which is a machine that spins at a high rate to cause the contents of your blood to separate. A lab specialist will add a special anticoagulant to keep your blood from clotting.
When the test tube is taken out of the centrifuge, it will have settled into three parts:
- red blood cells
- plasma, or the fluid in your blood
Each component will settle in a different part of the tube, with the red blood cells moving to the bottom of the tube. The red blood cells are then compared to a guide that tells what proportion of your blood they make up.
While the laboratory that tests the blood sample may have its own ranges, generally accepted ranges for hematocrit depend on your gender and age. Typical ranges are as follows:
- adult men: 38.8 to 50 percent
- adult women: 34.9 to 44.5 percent
Children ages 15 and under have a separate set of ranges, as their hematocrit levels change rapidly with age. The specific lab that analyzes the results will determine the normal hematocrit range for a child of a certain age.
If your hematocrit levels are too low or too high, it can indicate various problems.
Low hematocrit levels may be a sign of:
- bone marrow diseases
- chronic inflammatory disease
- deficiencies in nutrients such as iron, folate, or vitamin B-12
- internal bleeding
- hemolytic anemia
- kidney failure
- sickle cell anemia
High hematocrit levels can indicate:
Before getting the test, let your doctor know if you’ve recently had a blood transfusion or are pregnant. Pregnancy can decrease your blood urea nitrogen (BUN) levels due to increased fluid in your body. A recent blood transfusion can also affect your results. If you live at a high altitude, your hematocrit levels tend to be higher due to reduced amounts of oxygen in the air.
Your doctor will likely compare the results of your hematocrit test to the other parts of the CBC test and your overall symptoms before making a diagnosis.
A hematocrit test is not associated with any major side effects or risks. You may have some bleeding or throbbing at the site where the blood is drawn. Let your doctor know if you experience any swelling or bleeding that doesn’t stop within a few minutes of pressure being applied to the puncture site.