Trimethoprim interference in methotrexate assay by an enzyme inhibition assay kit.

Spurious methotrexate (MTX) concentrations in sera and cerebrospinal fluids from leukemia patients who were given trimethoprim (TMP) were estimated using an MTX assay kit which is based on its inhibition of dihydrofolate reductase. The interference of TMP with MTX was confirmed in the assay system. A concentration of 0.17 microgram/ml of TMP gave a value for an apparent MTX of 10 microM.