Microcytosis

To the Editor.— Steinberg and Dreiling's review of microcytosis (1983; 249:85) emphasizes the importance of differentiating iron deficiency from other causes of microcytosis for which iron therapy is both unwarranted and harmful. To complete the picture, it is important to emphasize that microcytic conditions may obscure a coexistent vitamin deficiency. When a microcytosis interferes with the expected vitamin deficiency-induced macrocytosis, the major stimulus to consideration of folate or vitamin B12deficiency is lost. This is especially unfortunate since folate deficiency often coexists with iron deficiency, and the metabolic demand for this vitamin is probably elevated in chronic disease states and thalassemia. Evaluation of serum levels of vitamin B12and folate despite the absence of macrocytosis seems to be cost-effective in moderate to severe anemia. Additional clues to vitamin deficiency in microcytic conditions may be provided by the finding of hypersegmented neutrophils on the peripheral smear, while target cells and an