A dimorphic blood film as a sign of the onset of iron-deficient erythropoiesis in megaloblastic anemia

Abstract

A dimorphic blood film is an important clue to a diagnosis of congenital or acquired sideroblastic anemia. However there are other more common causes, including the treatment of iron deficiency anemia, and blood transfusion in patients with a hypochromic microcytic anemia. We should like to draw attention to another cause. A 37-year-old woman presented with dyspnea and was found to have a macrocytic anemia: hemoglobin concentration (Hb) 54 g/l and MCV 149.6 fl. White cell and platelet counts were normal. A blood film confirmed marked macrocytosis with an absence of polychromasia. The neutrophils demonstrated hypersegmentation with neutrophils having up to eight nuclear segments. Hematinic testing showed red cell folate below the limit of detection, a normal serum B12 of 261 pg/ml (normal range 211-911), a high serum iron of 44 μmol/l (normal range 11-32), high transferrin saturation of 92% and high serum ferritin of 567 ng/ml (normal range 10-154). Following eight days of oral folic acid, her Hb had risen to 62 g/l and MCV had fallen to 132 fl. Her blood film had become dimorphic (left) and an erythrocyte size histogram done by impedance technology now showed a bimodal distribution (right). Her serum iron and transferrin saturation had fallen to 9 μmol/l and 19% respectively without any change in the total iron-binding capacity. These findings confirmed the suspicion from the blood film and automated instrument graphs that there was now iron-deficient erythropoiesis, for which there are two possible explanations: restoration of effective erythropoiesis by folic acid supplementation could have rapidly used up all available storage iron and unmasked iron deficiency or, alternatively, there could be functional iron deficiency due to the strong erythropoietic drive, as is observed sometimes in renal dialysis patients treated with erythropoietin.