Comparison of results of microbiologic and radioisotopic assays for serum vitamin B12 during pregnancy.

The serum vitamin B12 level falls progressively during pregnancy, when measured either by microbiologic assay or by a radioisotope dilution technique. The concentration of the vitamin assayed by radioisotope dilution was consistently higher than the value obtained by microbiologic assay. This discrepancy was greatest in early pregnancy and decreased as the duration of pregnancy increased. These results add further evidence to the hypothesis that changes in serum binders for vitamin B12 are of major significance in the pathogenesis of the observed fall in serum levels of the vitamin during pregnancy.