Cobalamin analogues are present in human plasma and can mask cobalamin deficiency because current radioisotope dilution assays are not specific for true cobalamin.

Since R protein binds cobalamin (vitamin B12) and cobalamin analogues, whereas intrinsic factor is highly specific for true cobalamin, we compared the serum cobalamin values obtained with these proteins in radioisotope dilution assays. With R protein, eight of 21 patients with cobalamin deficiency had serum cobalamin levels (mean, 204, range, 85 to 355 pg per milliliter) that overlapped with values for 74 normal subjects (mean, 576, range, 220 to 1230). With intrinsic factor, no patient values (mean, 36, range, less than 10 to 78 pg per milliliter) overlapped with the normal values (mean, 322, range, 130 to 785). Paper chromatography showed that these differences were due to the presence of cobalamin analogues. R protein constituted 51 to 85 per cent of the cobalamin-binding protein in 10 commercial serum cobalamin assay kits, which were said to contain "intrinsic factor". Human plasma contains cobalamin analogues that can mask cobalamin deficiency with current radioisotope dilution assays.