What is a complete blood count (CBC)?

 Dr. Kathleen Handal, MD
Emergency Medicine
A complete blood count (CBC) is a common blood test that provides a general picture of your blood levels. It’s typically ordered for complaints such as fatigue, weakness, inflammation, bruising, fever and bleeding. It is an automated count of the cells in your blood that includes information on the number, shape and size of your cells. A standard CBC includes eight tests, with the big five being:
  • White blood cell (WBC) -- count helps to identify infection, immune problems, cancer and leukemia. If the numbers are high or low, further testing is usually required.
  • Red blood cell (RBC) -- count helps to identify anemia if decreased.
  • Hemoglobin (HGB) -- measures the oxygen-carrying proteins in the blood. When low, it indicates blood loss and anemia which can be life-threatening.
  • Hematocrit (HCT) -- measures the percentage of red blood cells in a given volume of blood. Again, if the level is low, anemia is indicated.
  • Platelets are looked at to identify problems related to bleeding and clotting.
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A complete blood count (CBC) is an evaluation of a person’s blood cells. The CBC is often used to check for such disorders as anemia, infection, and many other diseases. It is actually a panel of tests that examines different parts of the blood and includes the following:

  • White blood cells, and their differential (the different types of white blood cells)
  • Red blood cell count, which is the actual number of blood cells per volume of blood
  • Hematocrit and hemoglobin which measures the bloods ability to carry oxygen and the percentage of red blood cells per volume of blood respectively
  • Platelet count, which measures the amount of platelets in a given volume of blood. Platelets help with blood clotting.


Paul M. Ehrlich, MD
Allergy & Immunology
The most comprehensive test of the immune system, the complete blood count (CBC) measures white blood cells, platelets, which are involved in clotting, and hematocrit -- the concentration of blood. The white blood cell count is often examined under a microscope to determine levels of these cells:

  • Lymphocytes, such as T and B cells, direct the entire immune system and make antibodies to fight infections.


  • Neutrophils, cells normally seen in pus, are the first line of attack against invading bacteria and other infections.


  • Eosinophils, usually found in very small quantities, are known to be important in allergic disease such as asthma or drug allergies. They are also elevated with conditions such as systemic lupus, cancer, or parasitic infection.


  • Basophils, similar to eosinophils, can also be found in allergic and other diseases.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.