Complete Blood Count (CBC): About This Test

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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 examination. 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.
  • Find anemia or an infection.
  • See how much blood has been lost if there is bleeding.
  • Diagnose diseases of the blood, such as leukemia or polycythemia.

How can you prepare for the test?

You do not need to do anything before having this test.

What happens during the test?

The health professional taking a sample of your blood will:

  • Wrap an elastic band around your upper arm. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

If this blood test is done on a baby, a heel stick may be done instead of a blood draw from a vein.

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.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse call line if you are having problems. It's also a good idea to keep a list of the medicines you take. Ask your doctor when you can expect to have your test results.

Where can you learn more?

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