Case Presentation: Vitamin B12 (B12) deficiency is a commonly encountered clinical condition that frequently presents with either macrocytic anemia or neurological symptoms. The objective of this report is to describe a rare case of severe B12 deficiency presenting with hemolytic anemia and pancytopenia

Discussion: A 27 year old male with no contributory medical history presented with complaint of fatigue and non-specific diffuse abdominal pain. Pertinent physical exam findings included conjunctival pallor and diffuse abdominal tenderness. Initial blood workup was significant for hemoglobin(Hb) 4.5(13.5-18)g/dL, high mean corpuscular volume, low white blood cell and low platelets. Additional evaluation of anemia showed elevated aspartate transaminase, rise in total and direct bilirubin, low haptoglobin, and elevated lactate dehydrogenase indicative of ongoing hemolysis. Subsequent peripheral blood smear showed macrocytosis, anisocytosis, and schistocytes. Given concern for autoimmune hemolysis, the patient was empirically started on corticosteroids. The evaluation for macrocytosis resulted in profoundly low B12 level <60 pg/mL, while his hemolysis workup, including direct antiglobulin test (DAT), disseminated intravascular coagulation panel, hepatitis panel, abdominal imaging, HIV screen and parvovirus B19 antibody was unremarkable. Also, he denied history of blood transfusions and reported cephalexin use for spider bite one month prior to presentation. He was started on B12 replacements, corticosteroids were discontinued, and he received multiple blood transfusions during hospitalization for goal Hb>7 g/dL. Bone marrow biopsy revealed hypercellular marrow with marked erythroid hyperplasia showing left shift to immature precursors, and dyserythropoiesis. Additional work up for B12 deficiency showed positive intrinsic factor antibody. Although drug-induced hemolysis secondary to cephalexin use is a possibility, the negative DAT test lead to the diagnosis of hemolytic anemia secondary to severe B12 deficiency with underlying pernicious anemia. The patient was discharged on day 7 and had complete resolution of pancytopenia by week 4.

Conclusions: This report highlights an uncommon initial presentation of severe B12 deficiency with hemolysis and pancytopenia, whereas commonly it presents with macrocytosis with or without neurological symptoms. B12 is a substantial element for DNA synthesis, normal RBC maturation, and thus its deficiency leads to ineffective or dysplastic erythropoiesis, large-oval erythroblasts. This macrocytosis leads to membrane instability, making erythrocytes prone to hemolysis. In general, macrocytosis is often overlooked. However, early identification and thorough workup for macrocytosis may play a crucial role in avoiding detrimental consequences.