Commercially-supplied binders for plasma cobalamin (vitamin B12), analysis--"purified" intrinsic factor, "cobinamide"-blocked R-protein binder, and non-purified intrinsic factor-R-protein binder--compared to microbiological assay.

Recent evidence (Kolhouse et al., N. Engl. J. Med. 299: 785-792, 1978) demonstrates that commercial cobalamin (Vitamin B12) radioassay kits contain nonspecific R-protein binding agents that can give falsely normal results in patients who are actually cobalamin deficient. We tested three kits: with "purified" intrinsic factor as the binder, with intrinsic factor and the nonspecific R-protein sites blocked with "cobinamide," and non-purified intrinsic factor-R-protein binder. Results with use of the first two compared well with those by a microbiological assay (Lactobacillus leichmannii) and are in harmony with clinical impressions.