Unreliability of radiodilution assays as screening tests for cobalamin (vitamin B12) deficiency.

Recent studies have demonstrated problems with the radiodilution assay method for measuring serum cobalamin (vitamin B12) levels. We have found the standard commercial radiodilution kit assay to be totally ineffective in screening for cobalamin deficiency. In a four-month period, 352 patients were screened in a routine clinical laboratory using this kit. None of the serum samples yielded a result below the given normal range of 200 to 1,100 pg/mL. However, when 42 patients with low-normal serum cobalamin levels (200 to 500 pg/mL) were retested by a combination of a modified radiodilution kit, a human intrinsic factor assay, and a microbiological (Lactobacillus leichmannii) assay, 16 (36%) were identified as having serum cobalamin levels that were abnormally low. The diagnosis of cobalamin deficiency may have been incorrectly dismissed in many patients on the basis of test results obtained with these kits, which are in widespread use.