The use of chicken serum for measurement of serum vitamin B12 concentration by radioisotope dilution: description of method and comparison with microbiological assay results

A new method for the measurement of serum vitamin B₁₂ concentration by radioisotope dilution is described in which chicken serum is used as the source of vitamin B₁₂ binding protein. The conditions for the assay were carefully selected on the basis of a thorough investigation of the factors affecting the critical steps in the radioisotope dilution assay for vitamin B₁₂. The assay obeys the principle of radioisotope dilution, and gives reproducible results. The normal range for the method (400 to 1020 pg/ml) is higher than any previously reported normal range for a radioisotope method. Excellent discrimination was obtained between normal subjects and those with pernicious anemia. The serum vitamin B₁₂ levels measured by the chicken serum radioisotope dilution assay in 596 subjects were compared with the results of either Euglena gracilis or Lactobacillus leichmannii microbiological assay. The chicken serum method consistently gave higher results than either microbiological assay, but the difference was especially great in many post gastrectomy, pregnancy, polycythemia vera and cord sera. A definite cause for these differences was not established.