The RDW-CV (RDW coefficient of variation) is inversely proportional to the mean cell volume (MCV). As a result, patients with microcytosis have elevated RDW-CV regardless of variation in cell size, while those with macrocytosis have lower RDW-CD values. By contrast, the RDW-SD (RDW standard deviation) is not influenced by the MCV and therefore may be preferable to use. An RDW-SD > 46 fL represents anisocytosis.