An Improved Radioisotope Dilution Assay for Serum Vitamin B 2 Using Hemoglobin - coated Charcoal

A radioisotope dilution assay for vitamin B12, using hemoglobin-coated charcoal, was modified in several respects, the most prominent of which was the use of cyanide extraction of sera. With these modifications, the B12 levels were significantly higher in sera from normal subjects and patients with folate deficiency, whereas sera from patients with pernicious anemia and other B12-deficient states were only minimally affected. The use of cyanide extraction thus resulted in a clearer differentiation of B12-deficient sera from other sera; all 23 patients with pernicious anemia had B12 levels below 140 pg/mI, whereas all of 85 normal and 218 folate-deficient subjects had B12 levels above 156 pg/mI. The microbiologic assay with Euglena grad/is was similarly affected by cyanide extraction of sera. Thus, it appears that the use of cyanide results in more complete extraction of B12 from serum proteins.