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RED BLOOD CELL INDICES Definition Red blood cell indices are measurements that describe the size and oxygen-carrying protein (hemoglobin) content of red blood cells. The indices are used to help in the differential diagnosis of anemia. They are also called red cell absolute values or erythrocyte indices. The relationships between the hematocrit, the hemoglobin level, and the RBC are converted to red blood cell indices through mathematical formulas. These formulas were worked out and first applied to the classification of anemias by Maxwell Wintrobe in 1934 Purpose Anemia includes a variety of conditions with the same outcome: a person's blood cannot carry as much oxygen as it should. A healthy person has an adequate number of correctly sized red blood cells that contain enough hemoglobin to carry sufficient oxygen to all the body's tissues. An anemic person has red blood cells that are either too small or too few in number. As a result, the heart and lungs must work harder to make up for the lack of oxygen delivered to the tissues by the blood. Anemia is caused by many different diseases or disorders. The first step in finding the cause is to determine what type of anemia the person has. Red blood cell indices help to classify the anemias. The relationships between the hematocrit, the hemoglobin level, and the RBC are converted to red blood cell indices through mathematical formulas. These formulas were worked out and first applied to the classification of anemias by Maxwell Wintrobe in 1934. The indices include these measurements: mean corpuscular volume (MCV); mean corpuscular hemoglobin (MCH); and mean corpuscular hemoglobin concentration (MCHC). MEAN CORPUSCULAR VOLUME The MCV is the average volume of the RBC in femtoliters (fL), or 1015L: MCV = Hct (%) X 10 / RBC count (1012-/L) Example: Hct = 45%, RBC count = 5.0x1012/L; therefore, MCV = 45.0x10 / 5.0 = 90fL Note: For the purpose of calculating only indices, units may disregarded to facilitate calculations. Cells of normal size (MCV is 80-100fL) are called normocytic, smaller cells are microcytic, and larger cells are macrocytic. These size categories are used to classify anemias. Normocytic anemias have normal-sized cells and a normal MCV; microcytic anemias have small cells and a decreased MCV; and macrocytic anemias have large cells and an increased MCV. Microcytic cells are found in: Patients with iron deficiency anemis. Thalassemia. Macrocytic cells are found in: Patients with liver disease or hypothyroidism When there is asynchrony in RBC maturation megaloblastic anemias). Folate and vitamin B12 deficiencies. (termed MEAN CORPUSCULAR HEMOGLOBIN (MCH) The MCH is the average weight of Hb in an RBC, expressed in the units of picograms (pg), or 10-12g: MCH = Hb (g/dL) X 10 / RBC count (1012/L) Example: Hb=16.0 g/fl. RBC count=5.0x1012/l. MCH=16.0x10 / 5.0 = 32.0pg The reference range for adults is 28-32pg. The MCH is not generally considered in the classification of anemias. MEAN CORPUSCULAR HEMOGLOBIN CONCENTRATION (MCHC) The MCHC is the average concentration of Hb in each individual erythrocyte. The units used are gram per deciliter (formerly referred to as a percentage). MCHC = Hb(g/dL) X 100 / Hct (%) Example: Hb =16 g /dl, Hct = 48%; MCHC=16 X 100 / 48 = 33.3g/dL Values of normochromic cells range from 32 to 37g/dL, those hypochromic cells are less than 32g/dL, and those of hyperchromic cells are greater than 37g/dL. Hypochromic erythrocytes occur in thalassemias and iron deficiency. Because there is a physical limit to the amount of hemoglobin that can fit in a cell, there is no hyperchromic category, a cell does not really contain more than 37g/dL of Hb, but its shape may have become spherocytic, making the cell appear full. An MCHC > 37g/dL should be scrutinized carefully for an error in Hb value. Key Terms Anemia A variety of conditions in which a person's blood can't carry as much oxygen as it should due to a decreased number or size of red blood cells. Hypochromic A descriptive term applied to a red blood cell with a decreased concentration of hemoglobin. Macrocytic A descriptive term applied to a larger than normal red blood cell. Mean corpuscular hemoglobin (MCH) A measurement of the average weight of hemoglobin in a red blood cell. Mean corpuscular hemoglobin concentration (MCHC) The measurement of the average concentration of hemoglobin in a red blood cell. Mean corpuscular volume (MCV) A measure of the average volume of a red blood cell. Microcytic A descriptive term applied to a smaller than normal red blood cell. Normochromic A descriptive term applied to a red blood cell with a normal concentration of hemoglobin. Normocytic A descriptive term applied to a red blood cell of normal size. Red blood cell indices Measurements that describe the size and hemoglobin content of red blood cells.