Radioassay for serum vitamin B₁₂ with the use of saliva as the vitamin B₁₂ binder

A rapid, easy radioassay for serum vitamin B₁₂ is described. Saliva, a readily available, rich source of vitamin B₁₂-binding protein, is used. The assay is reproducible, correlates satisfactorily with microbiological assay results, and clearly differentiates between B₁₂-deficient, normal, and high-B₁₂ sera. Poor exclusion by some hemoglobin-coated charcoals of saliva-bound B₁₂, in the absence of serum resulted in falsely low results. This problem can be corrected by acidification and redrying of the charcoal powder prior to coating or circumvented by additional coating of the hemoglobin-coated charcoal with vitamin B₁₂-saturated serum.