Dr Snow1 is right that the diagnosis of vitamin B12 or folate deficiency is no longer as straightforward as it once seemed. Although his review of available tests is useful, I must take issue with some of his assertions.
His list of nonmegaloblastic causes of macrocytic anemia fails to include the most common cause (especially in an elderly population), myelodysplastic disorders, particularly sideroblastic anemia. (Contrary to the classification in some textbooks, most acquired sideroblastic anemias are macrocytic overall.2) The inclusion of myelodysplastic disorders in the differential diagnosis of macrocytic anemia highlights other reasons for utility of the peripheral blood smear; findings such as dimorphic red blood cells, anisochromia, and pseudo–Pelger-Huët neutrophils can point to an alternative diagnosis to vitamin B12 or folate deficiency. Patients with macrocytic anemias have been inappropriately admitted and subjected to blood transfusion and extensive laboratory tests for rare hemolytic disorders because the basics of the peripheral blood smear review and reticulocyte count were neglected. Snow may be correct that nonhematologists do not comply with exhortations to examine the peripheral blood smear, but we do not need to condone or encourage the neglect of this most cost-effective test.
Rice L. Laboratory Diagnosis of Vitamin B12 and Folate Deficiency. Arch Intern Med. 1999;159(22):2745. doi: