What is a complete blood count?

A complete blood count (CBC) is a blood test that is commonly ordered by doctors. A CBC is often ordered as part of a complete physical or when your doctor thinks you might have a certain condition, such as an infection. A CBC may also be done to check on levels of prescribed medications in the body.

The test (which actually consists of several tests) gives details about three types of blood cells: red blood cells, white blood cells, and platelets. The CBC reports how many cells there are in the blood, and the physical characteristics of the cells, such as size, shape, and content.

What is a red blood cell, and what does a red blood cell count tell?

Red blood cells are the part of the blood that carry oxygen and carbon dioxide throughout the body. Red blood cells are made up of hemoglobin, which contains iron as part of its structure. The amount of oxygen that is combined with hemoglobin helps give the cells their red color. The hemoglobin carries oxygen to tissues and carbon dioxide (waste) away from tissues. The carbon dioxide leaves the body when it is exhaled through the lungs. Red blood cells are measured in millions per cubic millimeter (mil/mm3) of blood.

What is a white blood cell, and what does a white blood cell count tell?

White blood cells are the part of the blood that fight infections. White blood cells are measured in thousands per cubic milliliter (K/ml3) of blood.

A further test (white blood cell differential) might be done at the same time as the other blood tests. This test classifies the different kinds of white blood cells, which all have different jobs in keeping us healthy. The cells are: neutrophils (also called segs, PMNs, granulocytes, or grans), lymphocytes, monocytes, eosinophils, and basophils.

What kinds of disorders can be detected with a complete blood count?

Doctors may order a CBC when the patient has signs of infection, is weak or tired, or has inflammation (swelling), bruising, or bleeding. Some of these conditions may require treatment, while others may disappear on their own. Blood counts may also be affected by various medications and dietary deficiencies.

Abnormal CBC results help to diagnose:

  • Infections
  • Inflammation
  • Cancer
  • Leukemia
  • Autoimmune conditions (diseases in which the body's immune system attacks the body)
  • Bone marrow failure
  • Abnormal development of bone marrow
  • Anemia
  • Dehydration, fluid loss
  • Vitamin and mineral deficiencies
  • Thalassemia (a blood disorder in which the production of red blood cells is abnormal)
  • Effects of chemotherapy
  • Effects of certain antibiotics
  • Effects of a number of medications in long-term or even short-term use

Last reviewed by a Cleveland Clinic medical professional on 09/09/2014.


  • Lab Tests Online. Complete Blood Count Accessed 9/19/2014.
  • National Heart, Lung, and Blood Institute. Types of Blood Tests Accessed 9/19/2014.
  • National Institutes of Health. Understanding the Complete Blood Count (CBC) and Common Blood Deficiencies Accessed 9/19/2014.

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