Study interventions After a 7-day washout period, all consenting subjects were prescribed 400 mg of allopurinol in a single daily dose for 14 days. After a second 7-day washout period, all subjects were prescribed 80 mg febuxostat in a single daily dose for another 14 days. Twenty-four hour and first morning urine samples were collected at the end of the first washout period, and at the end of allopurinol and febuxostat treatment periods, respectively (days 7, 21 and 42). To minimize the potential adverse effect of dietary purine intake on the results, participants were asked to keep a food record while they collected the first 24 hr urine sample and adhere to the same diet when they collected the other two 24 hr urine samples. No further measures were taken to control dietary purine intake during the study period. At the end of the study, all patients were advised to return to their regular allopurinol dosing regimens.