How Does an Erythrocyte Become Macrocytic?

In addition to the MCV, two other red cell indices help determine RBC morphology. These include the mean corpuscular hemoglobin (MCH) and the mean corpuscular hemoglobin concentration (MCHC). In macrocytic anemias, one might think that it would hold more hemoglobin since the cell is larger. This, however, is not the case. The calculations of how these three indices are determined are shown in Table 1. The MCV is above the reference range in most macrocytic anemias, but the MCH and MCHC are usually within range.

By examining this information, it can be noted that the low hemoglobin in a patient with macrocytic anemia is due to a low red blood cell count, not due to insufficient hemoglobin per cell or unit volume. Most macrocytic anemias are due to faulty or ineffective erythropoiesis (red blood cell formation) in the bone marrow. However, the defect is not in the formation of the hemoglobin molecule. The defect involved is in the development of the cell itself. In many forms of macrocytic anemias, there is an error in how the RBCs are formed. Also, insufficient numbers of red cells are released into the circulation creating a low red cell count.

This course will examine why this happens and how these anemias are diagnosed.