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Complete Blood Count (CBC)

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 determines if there are any increases or decreases in your blood cell counts. Normal values vary depending on your age and your gender. Your lab report will tell you the normal value range for your age and gender.

A CBC can help diagnose a broad range of conditions, from anemia and infection to cancer.

The three basic types of blood cells

Measuring changes in your blood cell levels can help your doctor evaluate your overall health and detect disorders. The test measures the three basic types of blood cells.

Red blood cells

Red blood cells carry oxygen throughout your body and remove carbon dioxide. A CBC measures two components of your red blood cells:

Low levels of hemoglobin and hematocrit are often signs of anemia, a condition that occurs when blood 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 do the following.

  • Evaluate your overall health. Many doctors 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. Your doctor may regularly order CBCs to monitor your condition if you have been diagnosed with a disorder that affects blood cell counts.
  • 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 you can easily rollup.

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

What happens during a CBC?

During a CBC, a lab technician will draw blood from a vein, typically from the inside of your elbow or from the back of your hand. The test will take only a few minutes. The technician:

  1. cleans your skin with an antiseptic wipe
  2. places an elastic band, or tourniquet, around your upper arm to help the vein swell with blood
  3. inserts a needle in the your and collects a blood sample in one or more vials
  4. removes the elastic band
  5. covers the area with a bandage to stop any bleeding
  6. Label your sample and send it to a lab for analysis

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. Afterwards, you may have minor bruising, but it will clear up within a few days.

Most CBC results are available within a few hours to a day after testing.

For infants

In young infants, a 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 blood cell counts. Here are the normal results for adults, but different labs may deliver slight variations:

Blood componentNormal levels
red blood cell In men: 4.32-5.72 million cells/mcL
In women: 3.90-5.03 million cells/mcL
hemoglobinIn men: 135-175 grams/L
In women: 120-155 grams/L
hematocritIn men: 38.8-50.0 percent
In women: 34.9-44.5 percent
white blood cell count3,500 to 10,500 cells/mcL
platelet count150,000 to 450,000/mcL

A CBC is not a definitive diagnostic test. Blood cell counts that are too high or too low could signal a wide variety of conditions. Specialized tests are needed to diagnose a specific condition. Conditions that could cause an abnormal CBC and may require additional testing include:

  • 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 results. They may also order other tests to help further evaluate your condition and confirm a diagnosis.

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Written by: Danielle Mooreson: Jun 26, 2017

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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