Methotrexate in rheumatoid arthritis--a limited sampling strategy for estimation of the area under the plasma...

A limited sampling strategy for determination of the area under the plasma concentration versus time curve (AUC) of methotrexate (MTX) in patients with rheumatoid arthritis (RA), treated with weekly oral doses, has been validated. Stepwise linear regression analysis was used for optimal inclusion of data points in mathematical models to estimate AUC. A new plot for evaluation of the accuracy and precision of the estimated AUC values was introduced in the present study. By plotting the ratio of determined/estimated AUC values versus estimated AUC values, the influence of number of sampling points on the precision and accuracy of estimated AUC values was easily validated. Our results show that AUC values of MTX in RA patients can be estimated from a single plasma sample at 3 h or preferably, due to increased precision, by additional samplings at 5 and 1 h. A further increase of the number of sampling points increased the precision of the AUC estimates only to a minor extent. The accuracy of the estimated AUC values was independent of the number of sampling points. A limited sampling procedure can now be used for further studies on the relationship between MTX levels and its effects.