Risk factors associated with renal lithiasis during uricosuric treatment of hyperuricemia in patients with gout.

Objective: To find factors associated with the development of renal colic during uricosuric therapy.

Methods: We performed a prospective cohort followup study of patients with gout and no previous history of kidney stones who had been treated with uricosurics. Clearance of creatinine and urate, 24-hour urinary uric acid (UA), undissociated urinary UA concentration, 24-hour undissociated urinary UA, and pH and urine sediment were obtained. Cox proportional hazards regression analysis was used to identify variables associated with renal colic as the outcome. The rate of renal colic was compared with that of control patients receiving allopurinol who had no previous history of renal stones.

Results: We analyzed a 784 patient-year exposure from 216 patients: 206 with renal underexcretion of UA and 10 with normal excretion. There were 21 clinical events. Two variables showed increased risk hazard for developing lithiasis: clearance of UA at baseline and undissociated urinary UA concentration during followup. When only patients with underexcretion of UA were included in the analysis, undissociated urinary UA during followup remained the only statistically significant variable. Patients who showed an undissociated UA concentration < 20 mg/dl did not show an increase in the rate of lithiasis or events compared with patients receiving allopurinol.

Conclusion: Clearance of UA at baseline may be useful for selecting patients suitable for uricosuric treatment. The estimation of the concentration of undissociated urinary UA is useful for evaluating the risk of lithiasis during followup.