Pitfalls in the diagnosis of vitamin B12 deficiency by radiodilution assay
1979
The principle of radiodilution assays for vitamin B12 in plasma utilizes the concept that radioactive-labeled B12 competes with unlabeled B12 for sites on a fixed amount of binder. It is becoming evident, however, that the binders used in some assay kits are primarily the R protein type which are less specific for cyanocobalamin and hydroxycobalamin, the most biologically active forms of vitamin B12 in plasma. The binders can also detect biologically inactive cobalamin analogs. The misleading evidence has been examined in several studies. A comparison of 70 normal individuals with 43 patients with pernicious anemia was done by 2 separate microbiological and 2 radiodilution assays at 2 hospitals; the consistent results showed 10% of the anemic patients would have been diagnosed as not deficient in B12 according to radiodilution assay alone. Another study showed 6 of 45 patients with clinical evidence of B12 deficiency not defined by radiodilution assay. Analysis of 10 kits showed a variation of 10-49% in content of "intrinsic factor" binder; 51-82% was R protein, which could mean a variable portion of measured B12 was actually cobalamin analogs. A comparison of cobalamin plasma levels using hog and human R protein and intrinsic factor, and 2 case studies further support the need for continuing investigation of diagnosis of B12 deficiency.
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