Complete Blood Count (CBC) Test

What is a complete blood count?

A complete blood count (CBC) checks the blood cells in the blood. It measures the numbers of different types of blood cells, their sizes, and their appearance. It is a very common and useful blood test.

In general, the test measures 3 main components of blood:

  • Red blood cells (also called erythrocytes or RBCs). The test measures the number, size, shape, and appearance of the RBCs. It also measures the amount of hemoglobin in the RBCs. Hemoglobin carries oxygen from the lungs to the rest of the body. The part of the test called a hematocrit measures the percentage of the blood that is red blood cells.
  • White blood cells (also called leukocytes or WBCs). The test counts the number of white blood cells. White blood cells help your body fight infection. When each of the different types of white blood cells are counted, the test is called a CBC with differential. The most common types of white blood cells are:
    • Neutrophils (cells that respond to stress, such as bacterial infection)
    • Lymphocytes (cells that increase when the body fights a viral infection)
    • Eosinophils (cells that increase when you have allergies)
  • Platelets (also called thrombocytes). Platelets are not actually blood cells. They are fragments of large blood-forming cells. They are needed for blood clotting.

Why is this test done?

The CBC test may be done to check your overall health. It may also be done to check for:

  • Anemia (too few red blood cells)
  • Infection or other diseases
  • Problems with blood clotting
  • This test may also be done to see how well treatment for a disease or condition is working.

How do I prepare for this test?

Usually no preparation is needed for this test.

Talk to your healthcare provider if you have any questions about the test.

How is the test done?

Having this test will take just a few minutes. A small amount of blood is taken from a vein in your arm with a needle. The blood is collected in tubes and sent to a lab.

Ask your healthcare provider when and how you will get the result of the test.

What do the test results mean?

Red blood cell count:
Some of the reasons your red blood cell count may be higher than normal are:

  • You haven’t had enough fluids.
  • You are a smoker.
  • You have a disease called polycythemia vera that causes your body to make too many red blood cells.

A red blood cell count or hemoglobin level lower than normal is called anemia. The size of the red blood cells can help your healthcare provider know what might be causing the anemia.

White blood cell count
Some of the reasons your white blood cell count may be higher than normal are:

  • You have an infection.
  • You have inflammation, which is swelling and irritation in your body caused by an infection or disease.
  • You are taking certain medicines, such as prednisone.
  • You have a type of cancer called leukemia.

Your white blood cell count may be lower than normal if you have a viral infection, including the common cold, or if you are getting cancer treatment (chemotherapy).

Platelet count
Your platelet count may be higher than normal if you have an autoimmune disease (a disease that causes your body to mistakenly attack your own tissue), such as rheumatoid arthritis or Crohn’s disease. The platelet count can also go up with viral infections, but it will go back to normal as you recover.

Some of the reasons your platelet count may be lower than normal are:

  • You have a blood clotting problem, such as thrombocytopenia (an autoimmune disease)
  • You are taking certain medicines, such as sulfa drugs, quinine, heparin, or cancer drugs.
  • You have a blood infection or another serious illness.
  • You have an autoimmune disease, such as lupus.

What if my test result is not normal?

Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about your result and ask questions, such as.

  • If you need more tests
  • What kind of treatment you might need
  • What lifestyle, diet, or other changes you might need to make
Developed by RelayHealth.
Adult Advisor 2015.1 published by RelayHealth.
Last modified: 2014-06-02
Last reviewed: 2014-06-02
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
Copyright ©1986-2015 McKesson Corporation and/or one of its subsidiaries. All rights reserved.

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