Measurement of folates in human plasma and erythrocytes by a radiometric microbiologic method.

A radiometric microbiologic assay for the determination of folic acid in human plasma and red blood cells is described. The assay is based upon the measurement of 14CO2 produced from the metabolism of [1-14C] gluconate by Lactobacillus casei. The 14CO2 evolved was shown to be proportional to the amount of added DL-N-5-methyltetrahydrofolage (DL-N-5-methyl FH4). A total of 26 normal plasma and 57 blood hemolysates were assayed in parallel by this radiometric and the standard (turbidimetric) microbiologic assay. The correlation coefficients for the two assays were r = 0.96 for plasma and r = 0.98 for red-cell folate. Lyophilization of L. casei was found to simplify this radiometric assay by eliminating routine maintenance and culture of this microorganism.