Complete Blood Count (CBC): About This Test

What is it?

A complete blood count (CBC) is a blood test that gives important information about your blood cells, especially red blood cells, white blood cells, and platelets.

Why is this test done?

A CBC may be done as part of a regular physical exam. There are many other reasons that a doctor may want this blood test, including to:

• Find the cause of symptoms such as fatigue, weakness, fever, bruising, or weight loss.
• Check for anemia.
• See how much blood has been lost if there is bleeding.
• Diagnose polycythemia.
• Check for an infection.
• Diagnose diseases of the blood, such as leukemia.
• Check how the body is dealing with some types of drug or radiation treatment.
• Check how abnormal bleeding is affecting the blood cells and counts.
• Screen for high and low values before a surgery.
• See if there are too many or too few of certain types of cells. This may help find other conditions. For instance, too many eosinophils may be a sign of an allergy or asthma.

A blood count can give valuable information about the general state of your health.

How do you prepare for the test?

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How is the test done?

A health professional uses a needle to take a blood sample, usually from the arm.

What happens after the test?

• You will probably be able to go home right away.
• You can go back to your usual activities right away.