The RDW is an index of the variation in cell volume within the red cell population. It is a parameter provided by both impedance and flow cytometric analyzers. Mathematically, it is the coefficient of variation, i.e.,

Red cell populations with higher than normal RDW are termed heterogenous; those with normal RDW are homogeneous. For example, increased numbers of reticulocytes will cause an increased RDW. In some instances, the RDW is the first test result to increase with changes in red cell population sizes. For example, in early iron deficiency, there are only low numbers of microcytic red blood cells. This will increase the standard deviation and the RDW, but the mean cell volume is unchanged because there are insufficient numbers of microcytic cells to change the mean volume.

Which of these red cell histograms represents the population with the greater RDW? (Click within the image for explanation)

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Last Updated: June 2000