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Complete Blood Count

What Is A Complete Blood Count?

Blood is made up of fluid called plasma and many different types of blood cells. A complete blood count (CBC) is a test to find out the numbers of each type of blood cell.

Reason for the Test

There are many health conditions that can be found with a CBC. A CBC may be done:

  • As part of a routine check up
  • To check on symptoms that point to a health problem such as bleeding, anemia, infection, cancer, or allergies
  • To keep track of a current health problem
  • To see if treatment is working the way it is supposed to

Blood cell counts that are too high or too low may suggest a problem. The tests will also look at the shapes and size of blood cells.

Type of Sample Taken

A blood sample is taken from a vein in the arm.

Prior to Collecting the Sample

Tell your doctor:

  • About any medicines or supplements you are taking. They may affect the test results.
  • If you have had your spleen removed. This can affect red blood cell counts.

During the Sample Collection

You will be asked to sit. An area inside your elbow will be cleaned. A large band will be tied around your arm. The needle will then be inserted into a vein. A tube will collect the blood from the needle. The band on your arm will be removed. Once all the blood is collected, the needle will be removed. Some gauze will be placed over the site to help stop bleeding. You may also be given a bandage to place over the site. The process takes about 5 to 10 minutes.

For babies, the blood is taken with a heelstick. A small tool pierces the heel. A few drops of blood are taken and placed in a small tube. Cotton or a bandage is used stop the bleeding.

After Collecting the Sample

After the blood sample is taken, you may need to stay seated for 10 to 15 minutes. If you are lightheaded, you may need to stay seated longer. When you feel better, you can leave.

A bit of blood may ooze from the vein beneath the skin. It will cause a bruise. Firm pressure over the site after the needle is removed will decrease the chance of a bruise. A bruise will usually fade in a day or two.

Call your doctor right away if you have redness, swelling, lasting bleeding, or pain.


It will take many days to get a test result. Each blood cell type has a count that falls in a normal range. These change depending on your age.

Abnormal result may indicate:

  • For WBCs:
    • A high number may be caused by:
      • Infection
      • Inflammation
      • Allergies
      • Cancer or leukemia
    • A low number number may be caused by:
      • Infection
      • Chemotherapy
      • Problems with the bone marrow or immune system
    • Leukemias and lymphomas are linked to WBCs with an abnormal shape
  • For RBCs:
    • A high number may be caused by:
      • Dehydration
      • Diarrhea
      • Loss of blood plasma
      • Polycythemia
      • Burns
      • Smoking
      • Carbon monoxide exposure
      • Long-term lung disease
      • Kidney disease
      • Problems with the bone marrow
      • Certain cancers
    • A low number may show anemia, which may be caused by:
      • Low iron
      • Ulcers
      • Colon cancer
      • Sickle cell disease
      • Chemotherapy
  • For platelets:
    • A high number may be caused by Kawasaki disease
    • A low number be caused by:
      • Infection
      • Problems with the liver or spleen
      • Certain cancers
      • Problems from cancer treatment
      • Problems with the bone marrow

More testing may be needed to find the cause of abnormal CBC results.

Talk to your doctor about your test results. A test may point to an illness that you do not have. It can also miss an illness that you may have. The doctor will check your symptoms and all test results before making a diagnosis.


Complete blood count. Kids Health—Nemours Foundation website. Available at: https://kidshealth.org/en/parents/labtest4.html. Accessed February 7, 2017.

Complete blood count. Lab Tests Online—American Association for Clinical Chemistry website. Available at: https://labtestsonline.org/tests/complete-blood-count-cbc. Updated March 12, 2019. Accessed February 7, 2017.

Platelet count. EBSCO DynaMed Plus website. Available at:http://www.dynamed.com/topics/dmp~AN~T907153. Updated October 8, 2018. Accessed February 7, 2017.

Red blood cell count. EBSCO DynaMed Plus website. Available at:http://www.dynamed.com/topics/dmp~AN~T907140. Updated October 8, 2018. Accessed February 17, 2017.

White blood cell count. EBSCO DynaMed Plus website. Available at:http://www.dynamed.com/topics/dmp~AN~T907297. Updated October 8, 2018. Accessed February 7, 2017.

Last reviewed March 2019 by EBSCO Medical Review Board Nicole Meregian, PA