Oxypurinol and urate assay
Urate concentrations were measured using a Roche COBAS 6000 chemistry analyzer (Roche Diagnostics, Indianapolis, IN) using the enzymatic method with a limit detection of 0.2 mg/dL. The inter-assay coefficient of variation was 1.3% at 5.50 mg/dL and 2.0% at 9.67 mg/dL. Oxypurinol concentrations were measured as described in previous publication.25 None of the SU, serum oxypurinol, and urine oxypurinol concentrations were below the limit of quantification.