One of the most commonly ordered clinical laboratory tests, a blood count, also called a complete blood count (CBC), is a basic evaluation of the cells (red blood cells, white blood cells, and platelets) suspended in the liquid part of the blood (plasma). It involves determining the numbers, concentrations, and conditions of the different types of blood cells.

The CBC is a useful screening and diagnostic test that is often done as part of a routine physical examination. It can provide valuable information about the blood and blood-forming tissues (especially the bone marrow), as well as other body systems. Abnormal results can indicate the presence of a variety of conditions including anemias, leukemias, and infections, sometimes before the patient experiences symptoms of the disease.

A complete blood count is actually a series of tests in which the numbers of red blood cells, white blood cells, and...