To the Editor.
—The Schilling test for cobalamin (vitamin B12) absorption employs administration of free crystalline cobalamin and may fail to identify patients whose cobalamin deficiency is due to malabsorption of protein-bound cobalamin (the form that the vitamin exists in food).1-2 We2 have previously reported clinically significant cobalamin deficiency in five subjects with normal Schilling test results but abnormal results with a test employing protein-bound cobalamin. Carmel3 recently noted the evolution of classic pernicious anemia (malabsorption of free crystalline cobalamin normalizing with exogenous intrinsic factor, associated serum anti-intrinsic factor antibody) in a patient who previously had malabsorption noted only by means of a protein-bound cobalamin absorption test. Our experience suggests that the evolution noted by Carmel may be more common than appreciated. One of our five patients was recently retested for free crystalline cobalamin absorption because of a drop of his serum cobalamin level towards the