Serum vitamin B12 assays. How accurate are they?

Because of clinical evidence indicated that results of radioligand assays for serum vitamin B12 did not correlate with clinical outcome, the Ligand Assay Resource Committee of the College of American Pathologists (CAP) initiated studies of biochemical specificity in the 1980 CAP survey program. In 1980, the apparent cross-reactivity of inactive cobalamin analogues was 100% in most of the vitamin B12 assays. By 1981, probably because of method changes, the apparent cross-reactivity had drastically decreased, from 100% to 10%. Recovery data confirmed adequate chemical sensitivity for true cobalamin. When evaluated against the clinical requirements for diagnosing pernicious anemia, these performance characteristics are acceptable. Radioligand assays now used by CAP survey participants yield results equivalent to those of both radioligand assays using pure intrinsic factor and to bioassays. However, because these improved assays have been in use only a short time, detailed information about their clinical utility is not yet available. Therefore, generalizations about specific clinical decision levels are not warranted at this time.