Masked megaloblastic anemia.

In six patients, eight episodes of anemia associated with folic acid or vitamin B12 deficiency were unaccompanied by macrocytosis. Six of the eight episodes of anemia were complicated by illnesses of an inflammatory or infectious nature, two patients had iron deficiency, two appeared to have a thalassemia trait, and one had severe renal failure. In five of the eight episodes, erythropoiesis was not megaloblastic and there was insufficient anisocytosis or poikilocytosis to suggest an underlying vitamin deficiency state. Hypersegmented neutrophils were observed in all episodes, but a neutrophil lobe average of greater than 3.5 lobes per cell was observed only once, and in one patient, less than 5% of the circulating neutrophils were hypersegmented. Giant metamyelocytes, however, were present in the marrow in all of the episodes and provided an important clue to the presence of the vitamin deficiency state.