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Complete Blood Count

Medically reviewed by Last updated on Oct 31, 2022.

What is a complete blood count (CBC)?

A CBC is a commonly ordered blood test. It measures the levels of different cells in your blood. A CBC measures the following:

  • Red blood cells (RBCs) carry oxygen from the lungs and carbon dioxide (CO2) from tissues to the lungs. CO2 is a waste product that leaves the body through the lungs. The following may also be measured with RBC levels:
    • Hemoglobin (Hg) is the main part of the red blood cell. It helps carry oxygen and CO2.
    • Hematocrit (Hct) measures how much of your total blood count is RBCs. It is measured as a percentage.
  • White blood cells (WBCs) help fight infections. There are different kinds of WBCs. The levels of each kind of WBC give healthcare providers information about your immune system or infection.
  • Platelets are cells that help your blood clot and stop bleeding.

Why do I need a CBC?

  • A CBC may be done to check your overall health. A CBC may be done during a routine check-up or before surgery. A CBC will help your healthcare provider check for certain conditions. It will also make sure your blood can clot well enough before surgery or procedures.
  • A CBC may be done to diagnose a medical condition. You may need a CBC if you feel weak, tired, or bruise or bleed easily. You may also need a CBC if you have pain, fever, or swelling. A CBC will help diagnose the cause of these symptoms.
  • A CBC may be done to monitor a medical condition. You may need a CBC if you have been diagnosed with a blood or autoimmune disorder. A CBC will also help monitor your condition during an illness, after an injury, or after surgery.
  • A CBC may be done to monitor treatment. You may need a CBC to monitor your health if you take medicines that increase or decrease your blood cell levels. You may also need a CBC during cancer treatment. This is done to make sure your bone marrow is making enough healthy blood cells.

How do I prepare for the test?

You do not need to do anything to prepare for a CBC. The blood test may be done before or after eating and at any time of the day.

What medical conditions may a CBC help diagnose?

A CBC may be done with other tests to help diagnose certain medical conditions. Examples include:

  • Anemia (low red blood cell levels)
  • Thrombocytopenia (low platelet levels)
  • Sickle cell disease
  • Infection or an autoimmune disease
  • Heart disease
  • Allergies
  • Problems with how the body makes or gets rid of blood cells

Care Agreement

You have the right to help plan your care. Learn about your health condition and how it may be treated. Discuss treatment options with your healthcare providers to decide what care you want to receive. You always have the right to refuse treatment. The above information is an educational aid only. It is not intended as medical advice for individual conditions or treatments. Talk to your doctor, nurse or pharmacist before following any medical regimen to see if it is safe and effective for you.

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Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.