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CBC

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Definition:

A complete blood count (CBC) test measures the following:

• The number of red blood cells (RBCs)
• The number of white blood cells (WBCs)
• The total amount of hemoglobin in the blood
• The fraction of the blood composed of red blood cells ( hematocrit )
• The mean corpuscular volume (MCV) -- the size of the red blood cells

CBC also includes information about the red blood cells that is calculated from the other measurements:

• MCH (mean corpuscular hemoglobin)
• MCHC (mean corpuscular hemoglobin concentration)

The platelet count is also usually included in the CBC.

Alternative Names:

Complete blood count

How the test is performed:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.

How to prepare for the test:

There is no special preparation needed.

How the test will feel:

When the needle is inserted to draw blood, you may feel moderate pain, though most people feel only a prick or a stinging sensation. Afterward there may be some throbbing or bruising.

Why the test is performed:

The CBC is a screening test, used to diagnose and manage numerous diseases. The results can reflect problems with fluid volume (such as dehydration ) or loss of blood. It can show abnormalities in the production, life span, and rate of destruction of blood cells. It can reflect acute or chronic infection, allergies, and problems with clotting.

MCV, MCH, and MCHC values reflect the size and hemoglobin concentration of individual cells, and are useful in the diagnosis of various types of anemia .

• Review Date: 9/11/2006
• Reviewed By: Corey Cutler, MD, MPH, FRCP(C), Assistant Professor of Medicine, Harvard Medical School; Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA. Review provided by VeriMed Healthcare Network.

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