Methotrexate (MTX) is an antimetabolite of folic acid whose action mechanism consists of competitively inhibiting dihydrofolate reductase (DHFR), a key enzyme for cell proliferation. It is used for the treatment of childhood lymphocytic leukaemia and the monitoring MTX levels detects patients at risk and adjust their treatment. The purpose of this study is to validate the use of ARK™ methotrexate reagent for monitoring in the Cobas 6000 analyser from Roche Diagnostics®. Determinations were made of MTX levels in different serum samples using the Siemens Dade reagent used to date and the reagent to be used in the Cobas 6000 analyser from Roche Diagnostics®. The intra-series coefficients of variation ranged between 5.7, 1.5 and 1.9% for low, medium and high values, respectively; whereas the inter-series coefficients were 10.2, 7.5 and 8.3%. The linearity study was defined by the straight line: y=0.996x +0.148, r2= 0.989, with a 95% CI (0.983-0.994). The detection and functional sensitivity limits were 0.02 μmol/L and 0.184 μmol/L respectively. The correlation study resulted in a straight line y=0.960x+0.016, with an r2 of 0.980 (0.967-0.988). The linearity and correlation studies showed good results, with the imprecision study being acceptable. In view of the above, we conclude that the ARK™ methotrexate reagent can be used for monitoring MTX levels.
Marta Rico Rodríguez, María De-las Mercedes Calero Ruiz and Nelia Jiménez Valencia