Comparison of high-performance liquid chromatography and the Abbott fluorescent polarization radioimmunoassay in the measurement of methotrexate.

A modified high-performance liquid chromatographic (HPLC) technique for the assay of methotrexate is described and compared to the Abbott Fluorescence Polarization Radioimmunoassay. The reproducibility (coefficient of variation) at low concentrations was similar for the two assays: 8.1 and 8.5% for the Abbott and HPLC assay, respectively. The limit of detection of the two assays was also similar at 0.01 microM. The correlation coefficient for Abbott versus HPLC was 0.9833 with a gradient of 0.9545. Aspirin was the only drug that interfered with HPLC. Methotrexate's metabolite 7-hydroxymethotrexate did not interfere with the Abbott assay. Plasma half-lives were similar to oncology patients in the two rheumatological patients studied. The 7-hydroxymethotrexate half-life was 15 h.