Limitations of red blood cell distribution width (RDW) in evaluation of microcytosis

The red blood cell distribution width (RDW) has been proposed as an additional variable that would improve the initial classification of anemia. Microcytic anemias with an elevated RDW (greater heterogeneity) were used to distinguish iron deficiency from heterozygous thalassemia, which was said to have a normal RDW (more homogeneous). The authors attempted to classify their population of microcytic cases using the RDW as a major variable, but found only limited utility. While most of the iron-deficient cases had an increased RDW, almost one-half of the thalassemia cases also were classified as microcytic heterogeneous (increased RDW). The authors also found that target cells, erythrocytosis, and the ratios alone or in combination with the RDW were not specific in separating heterozygous thalassemia from iron deficiency. They conclude that a sequential evaluation (to include iron and hemoglobin studies) of cases of microcytosis is still needed.