A component of the complete blood count (CBC), the red blood cell (RBC) count determines the number of RBCs per cubic millimeters (expressed as the number of RBCs per liter of blood according to the international system of units [SI]). Because RBCs contain hemoglobin (Hgb), which is responsible for the transport and exchange of oxygen, the number of circulating RBCs is important. Although the life span of the normal RBC is 120 days, other factors besides cell age and decreased production can cause decreased values; examples are abnormal destruction due to intravascular trauma caused by atherosclerosis or due to an enlarged spleen caused by leukemia. The main sites of RBC production in healthy adults include the bone marrow of the vertebrae, pelvis, ribs, sternum, skull, and proximal ends of the femur and humerus. The main sites of RBC destruction are the spleen and liver. Erythropoietin, a hormone produced by the kidneys, regulates RBC production. Normal RBC development and function are also dependent on adequate levels of vitamin B₁₂, folic acid, and iron. A deficiency in vitamin E (α-tocopherol), which is needed to protect the RBC membrane from oxidizers, can result in increased cellular destruction. Polycythemia is a condition resulting from an abnormal increase in Hgb, hematocrit (Hct), and RBC count. Anemia is a condition resulting from an abnormal decrease in Hgb, Hct, and RBC count. Results of the Hgb, Hct, and RBC count should be evaluated simultaneously because the same underlying conditions affect this triad of tests similarly. The RBC count multiplied by 3 should approximate the Hgb concentration. The Hct should be within three times the Hgb if the RBC population is normal in size and shape. The Hct plus 6 should approximate the first two figures of the RBC count within 3 (e.g., Hct is 40%; therefore 40 + 6 = 46, and the RBC count should be 4.6 or in the range 4.3 to 4.9). (See monographs titled “Complete Blood Count, Hematocrit,” “Complete Blood Count, Hemoglobin,” and “Complete Blood Count, RBC Indices.”)