Cobalamin pseudodeficiency due to a transcobalamin I deficiency.
Adcock BB, McKnight JT.
Department of Family Medicine, College of Community Health Sciences, University of Alabama, Tuscaloosa, USA.

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.

[Note: The terms transcobalamin I, R-binder, and haptocorrin are synonymous and all refer to the same circulating plasma vitamin B12 binding protein. This protein is now commonly referred to as haptocorrin. Transcobalamin (or previously transcobalamin II) is the other binding protein, responsible for cellular up-take of vitamin B12. Current vitamin B12 assays measure cobalamin bound to both proteins. ]

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