The normal red cell distribution curve is Gaussian (bell-shaped) and the peak of the curve should fall within the normal MCV range of 80.0 - 100.0 fL. This is shown in Figure 1. The red cell distribution curve will get wider as the red cells vary more 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 red blood cells vary 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
The parameter RDW is not a determination of "normal" red blood cell size. For example, the cell population may be mostly large or mostly small cells and the RDW will be within the reference range. When the cells are observed on a smear, they will not be cells of typical size.
A population of cells that are of similar size, but not the size of normal RBCs, will produce a peak that is shifted in one direction or the other from the peak that represents normal RBCs. An RBC population with a low MCV will be shifted to the left on the histogram display as seen in Figure 3 on the right. As the MCV of a red blood cell population increases, the RBC distribution curve will move farther to the right on the display as seen in Figure 4 on the right.
When two distinct populations of red blood cells are present in a sample, the RDW will be increased and the red blood cell histogram may have more than one peak, as shown in Figure 5. The bottom image on the right is a peripheral blood smear made from the sample that produced the histogram in Figure 5. Note the two distinct RBC populations.Figure 5