Abstract
This case describes a patient with pancreatic exocrine insufficiency and persistently elevated serum vitamin B12 concentrations that were not due to supplementation or associated with hepatic or hematological pathology. Laboratory investigations suggested the presence of macro-B12 as the cause of this patient's raised serum vitamin B12. Macro-B12 is often formed when vitamin B12-vitamin binding proteins (transcobalamin and haptocorrin) complex with immunoglobulins. These molecules are thought to be biologically inert but their presence may lead to unnecessary investigations trying to eliminate other causes of elevated serum vitamin B12, such as solid tumors or hematological malignancies. If the presence of macro-B12 is suspected, the functional markers of B12 status, methylmalonic acid (MMA) or homocysteine, may be used preferentially to serum vitamin B12 to assess status.