BLOOD COUNT

determination of the number of red blood cells (RBC's, or erythrocytes) and white blood cells (WBC's, or leukocytes) in a given volume of blood. The readings vary with sex, age, physiological state, and general health, but a cubic millimetre of blood of a normal individual contains approximately 5,000,000 red cells and 7,000 white cells. A differential blood count is the enumeration of the number, or percent, of each type of white blood cell per 100 white cells counted; normally the white cells are about 55 percent neutrophils, 30 percent lymphocytes, and small percentages of eosinophils, basophils, and monocytes. A decrease in the number of red blood cells is usually associated with anemia, and an increase in the number of white blood cells with most infections or with leukemia. A blood count may also include a determination of the number of platelets, the volume by percent of red blood cells in whole blood (known as a hematocrit reading), the sedimentation rate of the red blood cells, and the hemoglobin concentration of the red cells.