The normal red cell distribution curve is Gaussian (bell-shaped), and the peak should fall within the normal MCV range of 80.0–100.0 fL, as shown in Figure 1. However, the red cell distribution curve will get wider as the red cells vary in size, as seen in Figure 2. Thus, a narrow distribution curve indicates a homogenous population of red cells; the wider the distribution curve, the more heterogenous the population of red cells.
Figure 1 An increase in the RDW is observed when the size of the RBCs varies within the red cell populations. This is physiologically a mix of different cell sizes and is known as anisocytosis. The wide peak would represent an RDW above the reference range. The population of cells would be variable in size. For example, some microcytic or small cells and some normal-size cells would result in a higher deviation and, hence, a higher RDW.
Figure 2
