[Detection of methotrexate in human serum. Comparison between an enzymatic and an enzyme-immunologic method]

High-dose methotrexate therapy with following citrovorum factor rescue necessitates the regular determinations of methotrexate (MTX) serum concentrations. A simple enzymatic assay by MTX induced inhibition of dihydrofolate reductase is described. The results gained with this assay are compared with those of a new enzyme immunoassay (MTX-EMIT). Between 5 X 10(-7) M and 1 X 10(-3) M the results of both tests are nearly the same, and the coefficient of correlation is 0,9980. At MTX concentrations below 5 X 10(-7) M the results of the MTX-EMIT lie nearly always higher than and differ considerably from those of the enzymatic method; the coefficient of correlation is only 0.7007. If it is possible to enhance the sensitivity of the MTX-EMIT, this test enables the very quick and easy determination of MTX serum concentrations in the whole range of clinical importance.