[Successful treatment with mecobalamin in a pernicious anemia patient presenting with false-normal serum vitamin B12].

An 81-year-old woman presented to our hospital with anemia. Complete blood counts revealed macrocytic anemia; however, serum vitamin B12 and folate levels were normal. Bone marrow aspiration revealed multilineage dysplasia, and the patient was initially diagnosed with refractory cytopenia and multilineage dysplasia subtype of myelodysplastic syndrome. However, blood smear revealed hypersegmented neutrophils and bone marrow aspiration showed remarkable megaloblastic changes of erythroid cells. Based on these findings, the patient was administered 1,500 µg mecobalamin per day on a trial basis. Three weeks after initiating mecobalamin, macrocytic anemia improved. Her hemoglobin levels were also normalized along with immediate resolution of peripheral blood dysplasia. The final diagnosis was pernicious anemia (PA) based on anti-intrinsic factor positivity and the efficacy of mecobalamin. Use of automated analyzers may be associated with falsely normal or falsely elevated vitamin B12 levels in the presence of anti-intrinsic factor antibodies. Our case suggests that trial administration of mecobalamin may be an important step to correctly diagnose PA associated with falsely normal or falsely elevated vitamin B12 levels, particularly when typical morphological features of PA are present.