A Radioassay for Serum Folate: Use of a Two-Phase Sequential-Incubation, Ligand-Binding System

Abstract

A folate binding-protein partially purified from milk was used as the binding ligand for a radioassay for serum folate. The procedure has a sensitivity of approximately 10 to 15 pg of N⁵-methyltetrahydrofolate, the natural folate in serum. The concentration of folate in normal serum was 8.24 ± 0.43 ng per milliliter (mean ± S.E.M.), and the concentration in serum from patients with clinical folate deficiency 1.38 ± 0.24 ng per milliliter. The variation of simultaneous duplicate determinations was 6.8 per cent. The variability of serums assayed on two different days was 10.6 per cent for levels above 0.5 and 15.3 per cent for those below 0.5 ng per milliliter. The radiochemical folate concentrations were approximately 1/2 the values obtained by a microbiologic assay using Lactobacillus casei. Where alcoholism and folate deficiency are common this radioassay is valuable because the results can be obtained in several hours and appropriate therapy quickly started.