What is a Complete Blood Count?

A complete blood count, or CBC, is a blood test that measures many different parts and features of your blood, including:

  • Red blood cells, which carry oxygen from your lungs to the rest of your body.
  • White blood cells, which fight infections and other diseases. There are five major types of white blood cells. A CBC test measures the total number of white cells in your blood. A different test called a CBC with differential measures the number of each type of these white blood cells.
  • Platelets, which stop bleeding by helping your blood to clot.
  • Hemoglobin, a protein in red blood cells that carries oxygen from your lungs to the rest of your body.
  • Hematocrit, a measurement of how much of your blood is made up of red blood cells.
  • Mean corpuscular volume (MCV), a measure of the average size of your red blood cells.

Other names for a complete blood count: CBC, full blood count, blood cell count

What is it used for?

A complete blood count is a common blood test that is often part of a routine checkup. Complete blood counts can help detect a variety of disorders including infections, anemia, diseases of the immune system, and blood cancers.

Why do I need a complete blood count?

Your health care provider may have ordered a complete blood count as part of your checkup or to monitor your overall health. The test may also be used to:

  • Help diagnose  blood diseases, infection, immune system disorders, or other medical conditions
  • Check for changes in an existing blood disorder

What happens during a complete blood count?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

Usually there is no special preparation necessary for a complete blood count. But if your provider ordered other tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

There is very little risk to having a blood test. You may experience slight pain or bruising at the spot where the needle went in, but most symptoms go away quickly.

What do the results mean?

A CBC counts the cells in your blood. There are many reasons your levels may not be in the normal range. For example:

  • Abnormal levels of red blood cells, hemoglobin, or hematocrit may be a sign of anemia, heart disease, or too little iron in your body.
  • Low white cell count may be a sign of an autoimmune disorder, bone marrow disorder, or cancer.
  • High white cell count may be a sign of an infection or a reaction to medicine.

If any of your levels are abnormal, it doesn’t always mean you have a medical condition that needs treatment. Diet, activity level, medicines, a menstrual period, not drinking enough water, and other factors can affect the results. Talk with your provider to learn what your results mean.