CBC (Complete Blood Count)

Written by Danielle Moores | Published on August 20, 2012
Medically Reviewed by George Krucik, MD

What Is a CBC?

A complete blood count, or CBC, is an easy and very common test that screens for certain disorders that can affect your health. A CBC measures several components of your blood and can help diagnose a broad range of conditions, from anemia and infection to cancer.

The Three Basic Types of Blood Cells

Changes in your blood cell levels can help your doctor to evaluate your overall health and to detect disorders. A CBC measures your blood to determine if there are any increases or decreases in your cell counts. Normal values vary depending on your age and your gender. Your lab report will indicate an appropriate normal value range.

The test measures the three basic types of blood cells.

Red Blood Cells

Your red blood cells work to carry oxygen to your body and remove carbon dioxide. A CBC measures two components of your red blood cells:

  • hemoglobin, the oxygen-carrying protein
  • hemocrit, the percent of red blood cells in your blood

Low levels of hemoglobin and hemocrit are often signs of anemia, or blood that is deficient in iron.

White Blood Cells

White blood cells help your body fight infection. A CBC measures the number and types of white blood cells in your body. Any abnormal increases or decreases in the number or types of white blood cells could be a sign of infection, inflammation, or cancer.


Platelets help your blood clot and control bleeding. When a cut stops bleeding, it’s because platelets are doing their job. Any changes in platelet levels can put you at risk for excessive bleeding and can be a sign of a serious medical condition.

When Is a CBC Ordered?

Your doctor may order a CBC as part of a routine checkup, or if you have unexplained symptoms such as bleeding or bruising. A CBC can help your doctor:

Evaluate your overall health.

Many physicians will order a CBC so they can have a baseline view of your health. A CBC also helps your doctor screen for any health problems.

Diagnose a health problem.

Your doctor may order a CBC if you have unexplained symptoms like weakness, tiredness, fever, redness, swelling, bruising, or bleeding.

Monitor a health problem.

If you have been diagnosed with a disorder that affects blood cell counts, your doctor may order regular CBCs to monitor your condition.

Monitor your treatment.

Certain medical treatments can affect your blood cell counts and may require regular CBCs. Your doctor can evaluate how well your treatment is working based on your CBC.

Getting Ready for a CBC

Make sure to wear a short-sleeved shirt, or a shirt with sleeves that can be easily rolled up.

Typically, you can eat and drink normally before a CBC. However, if the blood sample will be used for additional testing, your doctor may require that you fast for a specific amount of time before the test. Your doctor will give you specific instructions.

What Happens During a CBC?

During a CBC, a nurse or other health professional will draw blood from a vein, typically from the inside of your elbow or from the back of your hand.

The nurse will clean the surface of your skin with an antiseptic and place an elastic band, or tourniquet, around your upper arm to help the vein swell with blood. He or she will then insert a needle in the vein and collect a blood sample in one or more vials. The nurse will remove the elastic band and will cover the area with a bandage to stop the bleeding.

A blood test can be slightly uncomfortable. When the needle punctures your skin, you might feel a prick or pinching sensation. Some people also feel faint or light-headed when they see blood. You may experience minor bruising, but this will clear up within a few days.

The test will take only a few minutes. Afterward, your sample will be sent to a lab for analysis. Most results are available within a few hours to one day after testing.

For Infants

In young children, the nurse will typically sterilize the heel of the foot and use a small needle called a lancet to prick the area. The nurse will then gently squeeze the heel and collect a small amount of blood in a vial for testing.

What Do the Results Mean?

Test results will vary based on your specific cells count, and specific results can only be evaluated or diagnosed by your doctor. Generally, if blood cell counts are too high or too low, it could signal a wide variety of conditions, including:

  • iron or other vitamin and mineral deficiencies
  • bleeding disorders
  • heart disease
  • autoimmune disorders
  • bone marrow problems
  • cancer
  • infection or inflammation
  • reaction to medication

If your CBC shows abnormal levels, your doctor may order another blood test to confirm the results. He or she may also order other tests to help further evaluate your condition and confirm a diagnosis.

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