Red Cell Distribution Width (RDW) RDW is an expression of the homogeneity of the RBC population size. (Roll the cursor over the image.) A large RDW says there's a wide variation in the RBC diameters within the test pool.  It doesn't say the cells are large or small, rather that the population is not homogenous.  Younger cells are larger (reticulocytes). Older, and generally beat up, RBCs are smaller.