The parameter RDW does not determine "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 but will be consistent in size.
A population of cells 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 moved 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 from the sample that produced the histogram in Figure 5. Note the two distinct RBC populations.
Figure 5
