A simplified assay for methotrexate in biological fluids using a crude yeast lysate

Routine pharmacokinetic monitoring of methotrexate levels in biological fluids appears to be important for clinical applications involving large doses of methotrexate with leucovorin rescue. Recently several laboratories have independently described a competitive radiobinding assay that has the necessary sensitivity, precision and simplicity for this purpose. This study describes a modification of this assay method using a lysate of dry, active yeast as a source of binding activity. The assay is sensitive at blood concentrations of about 1 nM. No significant interference can be shown with naturally occurring folates. Specificity has also been shown by recovery experiments from serum and urine. Thus this modified assay matches the previously described radiobinding assays but with the advantage of using a methotrexate binding reagent that is readily available and quite inexpensive.