Cobalamin pseudodeficiency due to a transcobalamin I deficiency

Cobalamin (vitamin B12) deficiency warrants appropriate evaluation because cobalamin is necessary in certain biochemical functions. R-binder deficiency, which causes low cobalamin levels, is a rare and benign pseudodeficiency. If not further evaluated by determining levels of methylmalonic acid and homocysteine, however, such a patient would be given unneeded treatment. We report a case in which a patient has an R-binder deficiency, specifically transcobalamin I deficiency, with a low vitamin B12 level but no true vitamin B12 deficiency.