Pernicious Anemia With Spuriously Normal B12 Levels

We present a 29-year-old man admitted to our hospital with fatigue for two months of duration and recent palpitations, lightheadedness, blurred vision and nausea. Workup showed pancytopenia with severe macrocytic anemia, laboratory and blood smear features of hemolysis, low reticulocyte percentage and a negative direct Coombs test. B12 and folate levels were normal. As bone marrow aspirate was suggestive of megaloblastic anemia and upper endoscopy showed atrophic gastritis, we ordered homocysteine (elevated) and intrinsic factor (IF) antibodies (positive). The workup led to the diagnosis of pernicious anemia with spuriously normal B12 levels. Replacement therapy allowed a rapid recovery. We highlight that the presence of IF antibodies can interfere with the competitive binding assays commonly used to measure B12 levels.