Complete Blood Count
Medically reviewed by Drugs.com. 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
- Problems with how the body makes or gets rid of blood cells
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