Methotrexate serum-level determinations during low-dose therapy of rheumatoid and psoriatic arthritis

In 29 patients, 21 suffering from psoriatic arthritis and eight patients suffering from rheumatoid arthritis, methotrexate serum-levels were determined by means of radioimmunoassay. The aim of the investigation was to recognize an eventual dependence of the serum level of methotrexate on the total cumulative dose and to test the possibility of a concomitant therapy control. Beside the determinations of the serum levels of methotrexate, clinical examinations and laboratory tests were done at regular intervals. The values obtained showed no significant increase during the course of therapy compared to the values at the beginning of the treatment. Likewise no correlation to the total cumulative dose, the clinical picture or to the occurrence of side-effects could be found. Nor could any relationship between the changing of laboratory parameters and the methotrexate serum-levels be observed. No differences appeared in the methotrexate serum-levels during therapy of either rheumatoid or psoriatic arthritis patients. In conclusion it seems impossible to monitor a low-dose methotrexate therapy by continuous determinations of the serum levels of the drug.