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 DEAE cellulose is used to separate free and bound B₁₂, was compared (194 sera) with a packaged kit, in which intrinsic factor is used as binder and polysaccharide 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/ml; all B₁₂ deficient patients had values of < 165 pg/ml. In a separate study, 38 patients with documented (clinical and laboratory) B₁₂ deficiency had mean serum values of < 150 pg/ml by the reference assay, 250 pg/ml by the kit. For the kit, the normal range was 250-1100 pg/ml. All patients with serum values of < 150 pg/ml by kit assay were B₁₂ deficient; false positives were seen in the 150-250 pg/ml range.