An Improved Radioisotope Dilution Assay for Serum Vitamin B12 Using Hemoglobin-coated Charcoal

A radioisotope dilution assay for vitamin B₁₂, using hemoglobin-coatedcharcoal, was modified in several respects, the most prominent of whichwas the use of cyanide extraction ofsera. With these modifications, the B₁₂levels were significantly higher in serafrom normal subjects and patients withfolate deficiency, whereas sera frompatients with pernicious anemia andother B₁₂-deficient states were onlyminimally affected. The use of cyanideextraction thus resulted in a clearerdifferentiation of B₁₂-deficient serafrom other sera; all 23 patients withpernicious anemia had B₁₂ levels below 140 pg/ml, whereas all of 85 normal and 218 folate-deficient subjectshad B₁₂ levels above 156 pg/ml. Themicrobiologic assay with Euglenagracilis was similarly affected by cyanide extraction of sera. Thus, it appears that the use of cyanide resultsin more complete extraction of B₁₂from serum proteins.

Submitted on June 28, 1971 Revised on September 21, 1971 Accepted on September 23, 1971