Analytical evaluation of a folate radioassay: application to serum and erythrocyte measurements.

Many commercial folate radioassay kits have proven to be unsatisfactory in the past. It has been found that the Schwarz/Mann Iodine-125 folate radioassay kit permits a rapid measurement of folic acid requiring only a single incubation at pH 9.3. At this pH, the relative affinity of pteroylglutamic acid (FA) and N-5-methyltetrahydrofolic acid (MTFA) for the milk binder is the same. Additionally, the radioassay was compared to the L. casei microbiological assay. Regression analysis of 42 patients' sera, folate concentration 0.9 to 14 mug per liter, gave a correlation coefficient of 0.963. At folate concentration greater than 14 mug per liter, results with the radioassay in many instances were lower than those with the microbiological assay. In some cases the discrepancy was as much as 50 percent. However, the recovery of pure FA, MTFA and mixtures of the two at concentrations of 20 to 40 mug per liter was satisfactory by radioassay. This would indicate that the microbiological assay is inaccurate at higher folate concentrations. It has been shown that the Schwarz/Mann folate kit is accurate, precise, unaffected by icterus and lipemia. It is, therefore, suitable for both serum and red blood cell folate assay. A discrepancy has been observed between serum and red cell folate concentration; consequently, a simultaneously assay of both folates is suggested.