Comparison of two methods for radioassay of vitamin B12 in serum.

A reference radioassay for vitamin B12 in serum, in which human serum is used as binder and DEAEcellulose is used to separate free and bound B12, was compared (194 sera) with a packaged kit (“Phadebas B12 Test”), in which intrinsic factor is used as binder and poiysaccharide for separation. The correlation coefficient was 0.949. With the kit, added B12 was identifiable, proportional dilution characteristics were excellent, and serial reproducibility was good. The normal range for the reference assay was 200-1000 pg/mi; all B12-deficient patients had values of <165 pg/mi. In a separate study, 38 patients with documented (clinical and laboratory) B12 deficiency had mean serum values of <150 pg/mi by the reference assay, 250 pg/mi by the kit. For the kit, the normal range was 250-1100 pg/mi. All patients with serum values of <150 pg/mi by kit assay were B12 deficient; false positives were seen in the 150-250 pg/mI range.