Assay of Therapeutic Doses of Methotrexate in Body Fluids of Patients with Psoriasis

A microbiologic technique for the assay of methotrexate (MTX) in urine, serum, erythrocytes, feces, and skin is described. The accuracy of the method equals that of routine microbiologic assays of folic acid. Important differences in serum MTX levels in psoriatic patients during the 24 hr after standardized intravenous and intramuscular administration were demonstrated. Repeated intravenous doses tended to be cleared from the blood uniformly.After oral doses many patients achieved peak serum levels within 2 hr, with fall of level by 4 hr. Others achieved lower levels and responded less well clinically. Persistence of high serum levels at 24 and 48 hr did not confer obvious clinical benefit or necessarily give rise to toxicity. The pattern of clearance from serum was not influenced by serum albumin levels or by renal function when the creatinine clearance was greater than 50 mI/min. However, impaired renal function was clearly correlated with slow clearance.Routine measurement of MTX blood levels is of value in patients with suspected malabsorption or partial renal failure