What Do Blood Tests Show?

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The most common blood test, the complete blood count, or CBC, measures red blood cells, white blood cells, hemoglobin, hematocrit, platelets and mean corpuscular volume of the blood, according to the National Heart, Lung and Blood Institute. This test is often used to check symptoms and diagnose a wide variety of conditions, such as anemia, infections, blood cancers, clotting problems and immune system disorders.