Erroneously low results due to high nonspecific binding encountered with a radioassay kit that measures true serum vitamin B12.

Subsequent to recent disclosures that low serum vitamin B12 results can appear normal using many of the current radioassay methods because of the presence of cobalamin (vitamin B12) analogues in human serum, a number of radioassay kits have become available with modified intrinsic factor preparations and claims of measuring "true" vitamin B12. While evaluating two of these kits, one with R protein activity blocked and the other with R protein removed, the possibility was discovered that normal results could appear low owing to high nonspecific binding encountered with one of the methodologies. The problem of nonspecific binding was apparently due to incomplete inactivation of serum endogenous binding proteins and inadequate separation of free and bound radioisotope. The results demonstrate that "lower" vitamin B12 values are not necessarily "truer" values, and therefore caution and critical evaluation should temper the haste to change to commercial radioassay procedures that claim to provide "true" vitamin B12 levels.