Urinary N-acetyl-beta-D-glucosaminidase and serum creatinine concentrations predict impaired excretion of methotrexate.

We determined the risk of impaired excretion of methotrexate (MTX) in children with osteosarcoma, who also were receiving cisplatin, by analyzing urinary markers of renal tubular damage, as well as serum creatinine measured before each dose of MTX. MTX clearance was impaired in seven of the ten patients studied after cisplatin therapy. Patients with a urinary N-acetyl-beta-D-glucosaminidase (NAG) concentration of greater than 1.5 U/mmol creatinine or a greater than 50% increase in serum creatinine relative to the pretherapy level were approximately 30 times more likely to have MTX half-lives greater than 3.5 hours than were patients with lower values for these markers; MTX clearance was always impaired if both markers were elevated. If neither urinary NAG nor serum creatinine concentrations increased, the risk of impaired MTX excretion was negligible. Our findings demonstrate that urinary NAG and serum creatinine levels, measured before MTX administration, can be used to identify patients who will have difficulty in clearing the drug.