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What is a CBC?

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Updated April 29, 2009

Red blood cells

Red blood cells seen under the microscope

Photo © A.D.A.M.
Definition: In a complete blood cell count (CBC), a sample of blood is taken and examined under a microscope. To obtain the blood sample, a lab technician inserts a needle into a vein, usually in your arm in the inside of your elbow. Sometimes, especially in babies, the blood sample may be taken from a vein somewhere else on the body, like the back of the hand. The technician puts the blood sample in a vial and it is sent to the laboratory for analysis.

Blood is made up of three types of cells: red cells, white cells, and platelets. The CBC measures:

  • the number of red blood cells
  • the number of white blood cells
  • the number of platelets
  • the amount of hemoglobin in the blood
  • the fraction of the blood composed of red blood cells (this is called "hematocrit")
  • the size of the red blood cells (called "mean corpuscular volume," or MCV)
  • information about the size and hemoglobin content of individual red blood cells
The information from the CBC can help diagnose and manage diseases. Many different diseases can cause changes in the numbers and types of blood cells. For example, if anemia is present, there may be lower than normal numbers of red blood cells, while an infection may cause higher than normal numbers of white cells to be present. A disease like aplastic anemia can cause low numbers of all three types of blood cells.
Also Known As: complete blood count
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