Measurements of sub-nanomolar concentrations of unmetabolised folic acid in serum

We describe a combined HPLC/microbiological assay procedure for the sub-nanomolar analysis of unmetabolised folic acid (pteroylglutamate) in human serum. This metabolically unaltered form of the vitamin arises following the consumption of folic acid either in supplemental form or in fortified foods. Following HPLC separation of folic acid from other folate derivatives the folic acid fraction was concentrated by C18 Sep-Pak cartridges and assayed by Lactobacillus casei microbiological assay. The present assay allows the quantitation and kinetic analysis of the effects of consumption of folic acid.