What Is Hematocrit?

Hematocrit is a measurement that determines the percentage of blood made of up red blood cells (RBCs). Hematocrit rises when the number of RBCs is increased or the amount of plasma is reduced, such as with dehydration. It falls when the production of RBCs are decreased, the destruction of RBCs are increased, or there is blood loss.

Reason for the Test

The test is most often performed when anemia, polycythemia, or dehydration is suspected.

Type of Sample Taken

• Blood sample from a vein in the arm
• Fingerstick
• Heelstick—in newborns

Prior to Collecting the Sample

No special preparation is required prior to the test.

During the Sample Collection

If your blood sample is collected from a vein in your arm, you will be asked to sit. An area inside your elbow will be cleaned with an antiseptic wipe. A large band will be tied around your arm. The needle will then be inserted into a vein. A tube will collect the blood from the needle. The band on your arm will be removed. After all the blood is collected, the needle will be removed. Some gauze will be placed over the site to help stop bleeding. You may also be given a bandage to place over the site. The process takes about 5-10 minutes.

If your sample is taken with a fingerstick, a small metal scalpel is used to pierce your finger. The blood sample may be collected from a heelstick in infants. A heelstick uses a small metal scalpel to pierce the infant's heel to collect a few drops of blood. The blood will be collected into a small tube. Cotton or a bandage may be applied to help stop the bleeding.

After Collecting the Sample

After a blood sample is collected, you may need to stay seated for 10-15 minutes. If you are lightheaded, you may need to stay seated longer. When you feel better, you can leave.

In some cases, a bit of blood may ooze from the vein beneath the skin and cause a bruise. The risk of bruising can be minimized by placing firm pressure over the puncture site. A bruise will usually resolve in 1-2 days.

After the sample has been taken, the following complications may rarely occur at the site of the blood draw:

• Redness
• Swelling
• Persistent bleeding or discharge
• Pain

Call your doctor right away if you experience any of these complications.

Results

You should receive the results within a few days.

A hematocrit value below 20% indicates severe anemia. Mild anemias are below 30%

Reasons for a low hematocrit may include:

• Vitamin or mineral deficiencies
• Recent bleeding
• Red blood cell (RBC) abnormalities, like sickle cell anemia
• Hemolytic disease
• Cirrhosis
• Cancer and chemotherapy
• Kidney failure
• Chronic disease
• Bone marrow problems—decreased production of RBCs in bone marrow
• Enlarged spleen

A very high hematocrit value is over 60%, which may be caused by:

• Dehydration
• Polycythemia
• High altitudes
• Burns
• Excessive production by the bone marrow
• Poor lung function with low oxygen levels

Many factors can affect the reliability of lab tests. A test may suggest an illness that actually does not exist. This is called a false positive. A test may also miss an illness that actually does exist. This is called a false negative.

A doctor will consider the results from many tests and your symptoms before making a diagnosis. It is important to discuss these results with your doctor before making any conclusions.