Tissue vitamin B-12 assay by a radioisotope dilution technique

A method is described for the measurement of vitamin B-12 (B-12) in solid tissues by radioisotope dilution (RID) assay. The method is a modification using double extraction of a serum B-12 RID assay which uses chicken serum as the B-12 binder. The method was developed and tested using human and bat liver specimens. Double extraction was shown to be more efficient than single extraction, and resulted in complete release of endogenous liver [⁵⁷Co]B-12 administered to bats. Results using the RID assay in 16 human and 17 bat liver specimens were compared with those obtained using the Lactobacillus leichmannii microbiological assay. Correlation was good, but the RID assay gave higher results. Lower results using the microbiological assay appear to be due to inadequate extraction of B-12 from tissues.