blood count, method for determining the number of red (erythrocytes) and white (leukocytes) blood cells in a certain volume of blood. This test can be used as a preliminary step in diagnosing some diseases. Leukemia, for instance, causes an increase in the white blood cell count, while a decrease in the number of red blood cells is associated with anemia. A determination of the percentage of various types of white blood cells (neutrophils, lymphocytes, eosinophils, basophils, and monocytes) is called a differential blood count. A complete blood count (CBC) is an extended series of tests to determine such additional factors as the hemoglobin concentration of red blood cells and the amount of platelets in the blood.