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.

How is a complete blood count test performed?

In order to perform the CBC panel of tests, blood must be drawn from the patient. In adults, the blood is usually obtained from a vein in the arm. In infants, the blood is usually taken from the heel.

What should I do to prepare for a complete blood count test?

You do not have to do anything to prepare for a CBC.

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 other measurements are shown by a complete blood count?

  • Hematocrit (HCT)—the percentage of red blood cells in the total volume of blood
  • Platelet count—the number of platelets in thousands per cubic milliliter (K/ml3) of blood. Platelets form clots in order to stop bleeding.
  • Mean platelet volume (MPV)—the average size of platelets. This is important because new platelets are bigger than older ones, and a higher MPV indicates a higher platelet output.
  • Mean corpuscular hemoglobin (MCH)—the average amount of hemoglobin in a red blood cell.
  • Mean corpuscular hemoglobin concentration (MCHC)—the average concentration of hemoglobin in a red blood cell. The MCHC gives the health care provider an impression of the pallor of the cell; for example, very pale to very dark red. The degree of paleness may help in establishing a diagnosis.
  • Mean corpuscular volume (MCV)—the average size of red blood cells. "Macrocytic" describes a state in which red blood cells are bigger than normal; "microcytic" refers to the state in which red blood cells are smaller than normal. The average size of the red blood cells may help in establishing a diagnosis.
  • Red blood cell distribution width (RDW)—the amount of variation in the size of the red blood cells.

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


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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 9/9/2014…#4053