Evaluation of a radioassay for serum folate and the effects of ascorbate and methotrexate.

In this study a competitive binding assay for serum folate is evaluated with special attention to storage conditions of the serum samples and the effect of methotrexate on the assay results. Average serum folate levels were 31 nmol/1 +/- 8 (S.D.) in 59 nromal sera and 19 nmol/1 +/- 12 (S.D.) in 354 sera from hospitalized patients of which 43 values were lower than 9 nmol/1. Serum folate appeared to be stable at - 20 degrees C for at least 5 weeks. The addition of ascorbate, which is said to stabilize serum folate, caused a non-specific increase of the measured folate. Methotrexate acted competitively with the tracer folate in the assay sytem when added in vitro and elevated the folate readings. The same occurred in the serum of a patient after intravenous methotrexate.