Methotrexate measurements in plasma: comparison of enzyme multiplied immunoassay technique, TDx fluorescence polarization immunoassay, and high pressure liquid chromatography.

One hundred nine patient plasma samples were examined for methotrexate (MTX) levels by enzyme multiplied immunoassay technique (EMIT), fluorescence polarization immunoassay (TDx), and high pressure liquid chromatography (HPLC). EMIT analysis was performed twice within a time span of 18 months. All three methods measure MTX with a high degree of specificity, sensitivity, and precision. There was no evidence of decay of MTX concentrations in samples stored at -20 degrees C for 1.5 years. EMIT, TDx, and HPLC are adequate methods for MTX quantification in the clinical laboratory.