Radioimmunoassay for pteroylglutamic acid.

I describe a radioimmunoassay for specific determination of pteroylglutamate in plasma, erythrocytes, and urine. Antiserum was raised in rabbits immunized with a conjugate of pteroylglutamic acid and methylated bovine serum albumin. The lowest detection limit of the assay was 0.1 microgram/L; the coefficient of variation ranged from 4.01 to 14.63%. No significant interference was demonstrated from tetrahydrofolic acid, 5-methyl-tetrahydrofolic acid, or methotrexate. Under normal sampling and storing conditions dihydrofolic acid was oxidized to pteroylglutamic acid and estimated as such in the assay. The clinical applicability of the assay is demonstrated by measurements of pteroylglutamate absorption rates after oral doses of pteroylglutamic acid or pteroyltriglutamic acid and by the estimation of the accumulation of pteroylglutamate and dihydrofolate in plasma during therapy with methotrexate.