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How the CBC Test Determines the Number of Red Cells, White Cells and Platelets

written by: Leigh A. Zaykoski•edited by: Leigh A. Zaykoski•updated: 12/21/2008

Doctors use a wide range of blood tests to determine what is causing a patient's symptoms. The CBC blood test is one of the most basic blood tests, and can help doctors determine if a patient has anemia, infection, or other medical conditions.

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    Uses

    The CBC blood test is a panel of tests that help doctors to examine each part of the blood. The test can be used in several different ways. In an emergency room situation, the CBC blood test can be used to determine if the patient has a severe infection or is anemic. The levels of specific blood components can also be used to determine if a patient has other medical conditions (American Association for Clinical Chemistry).

    The CBC blood test can also be used as a diagnostic tool for outpatients or patients who have been admitted to the hospital with chronic conditions. For example, someone who is experiencing fatigue may have the CBC blood test done to determine if they are anemic.

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    CBC Blood Test Components

    The CBC blood tests measures a number of different blood components that are responsible for many important functions. White blood cells are counted and expressed as a number of cells per volume of blood. Both low and high levels of white blood cells can indicate an abnormality. The level of red blood cells in the blood can be indicative of a number of medical conditions. This value is expressed as the number of red blood cells in a volume of blood. Hemoglobin is a measure of the protein that carries oxygen in the blood. Low hemoglobin levels can indicate anemia, but high levels can also indicate medical problems. The platelet count can give doctors an idea if a patient has bleeding or clotting disorders. Other values tested by the CBC blood test include white cell differential, hematocrit, mean corpuscular volume, mean copuscular hemoglobin, and red cell distribution width (American Association for Clinical Chemistry).

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    CBC Blood Test Results

    Each laboratory has its own reference range that dictates what values are considered normal. The white blood cell count may be low due to autoimmune conditions and bone marrow failure, or due to some medications. The white blood cell count may be high due to infection, cancer, and leukemia. The red blood cell count can be low due to anemia and high when the patient has diarrhea and dehydration. The hemoglobin and hematocrit will also be low due to the presence of anemia. The platelet count can be decreased due to bleeding disorders, Lupus, and some types of anemia. The platelet count can be increased due to clotting disorders (American Association for Clinical Chemistry).

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    CBC Blood Test Reference Materials

    The following resources were consulted in the preparation of this article on the CBC blood test.

    American Association for Clinical Chemistry. Lab Tests Online. "CBC." Retrieved December 2, 2008. Available: American Association for Clinical Chemistry