Use of an improved E. coli method for the measurement of cobalamin in serum: comparison with the E. gracilis assay results.

Owing to the higher serum cobalamin results that are obtained by R-binder radioisotopic dilution assay compared to microbiological assays (E. gracilis and L. leichmannii) it was suggested that serum contained a cobamide(s) that could not be detected by the more specific microbiological assays and that a much less specific test organism, which responds to most naturally occurring cobamides, such as the cobamide-dependent E. coli mutant, might respond to these cobamide(s) in serum. In an attempt to investigate this possibility an improved and simplified E. coli assay for the measurement of cobamide in serum was developed. The method is described, and the results obtained in normal subjects, in patients with megaloblastic anemia, and in anaemic pregnant women not suffering from megaloblastic anaemia are reported and compared with E. gracilis assay results.