The laboratory diagnosis of megaloblastic anemias.

The diagnostic approach to megaloblastic anemia involves four usually sequential steps. The first step, recognition of megaloblastosis, requires attention to altered blood cell size and morphology. These changes may sometimes be subtle or masked. The cornerstone of the second step, identification of the specific vitamin deficiency, is assay of serum vitamin B(12) and folic acid levels, although they may occasionally be misleading. The third step, identification of the specific disease entity responsible for the vitamin deficiency, generally revolves around tests of absorption and gastric function. The fourth step, reevaluation after replacement therapy, is often not thought of as a diagnostic step but carries important diagnostic implications and is sometimes the only way in which coexisting abnormalities can be unmasked and identified.