Are ligand assay methods specific for cobalamin? A survey report.

To determine whether or not laboratories participating in the ligand assay survey of the College of American Pathologists can measure cobalamin (vitamin B-12) in the presence of cobalamin analogues, either cobalamin or cobinamide (a model for the non-physiological analogues of cobalamin) was added to pooled human serum, and aliquots of the various samples were submitted to the participants. Nine hundred and nineteen results that represented 13 commercially available methods were analyzed. For a sample containing 412 pg/mL. of cobalamin, the mean interference produced by 400 pg/mL of cobinamide, expressed as the apparent increase in the cobalamin concentration, was 26.5 pg/mL with a range of 18 to 42 pg/mL. The mean recovery of added cobalamin was 104% of the expected value.