Radioisotopic assay of vitamin B₁₂ in tissues

The present study delineates the application of the radioisotopic competitive-inhibition assay for the measurement of vitamin B₁₂ in tissues. The extraction of endogenous B₁₂ from tissues was shown to be simple and complete. Proportional dilution studies suggest that tissue factors do not interfere with the assay. Although some variability exists when multiple areas of an organ are sampled, the differences between B₁₂-deprived individuals are so wide that individual intra-organ variability is trivial. As this tissue B₁₂ assay is similar to the widely utilized radioisotopic assays for serum B₁₂, it is applicable for routine use.