#00012947

A 59-year-old woman with malaise was admitted to hospital with hemoglobin at 9.2 g/dL. Platelets were normal, leukocytes were 3.2 × 103/µL, reticulocytes were low, and mean corpuscular volume was 128 fL. The peripheral smear showed hypersegmentation and large polychromatophilic erythrocytes. A vitamin B12/folate deficiency was suspected. However, initial laboratory work showed vitamin B12 at 490 pg/mL (normal, 176-949 pg/mL) and folate at 10.3 ng/mL (normal, 2.6-16.0 ng/mL). Bone marrow showed megaloblastic erythroblasts, megaloblastic metamyelocytes with large bone-shaped nuclei, and mature neutrophils with hypersegmentation. Homocysteine and methylmalonic acid were elevated (65.7µM and 4846 nmol/L, respectively), indicative of a functional vitamin B12 deficiency. In addition, holotranscobalamin, the biologic available form of vitamin B12, was < 1 pmol/L. Antibodies against intrinsic factor were strongly positive. A diagnosis of vitamin B12 deficiency was still suspected despite the i