Objective To investigate gout flare rates based on repeated serum urate (SU) measurements in a randomised controlled trial of urate-lowering therapy (ULT), accounting for dropout and death.

Methods We performed a secondary analysis using data from Cardiovascular Safety of Febuxostat or Allopurinol in Patients with Gout, which randomised participants to febuxostat or allopurinol, titrated to target SU <6 mg/dL with flare prophylaxis for 6 months. SU was categorised as ≤3.9, 4.0–5.9, 6.0–7.9, 8.0–9.9 or ≥ 10 mg/dL at each 3–6 month follow-up. The primary outcome was gout flare. Poisson regression models, adjusted for covariates and factors related to participant retention versus dropout, estimated gout flare incidence rate ratios by time-varying SU category.

Results Among 6183 participants, the median age was 65 years and 84% were male. Peak gout flare rates for all SU categories were observed in months 0–6, coinciding with the initiation of ULT and months 6–12 after stopping prophylaxis. Flare rates were similar across SU groups in the initial year of ULT. During months 36–72, a dose–response relationship was observed between the SU category and flare rate. Lower flare rates were observed when SU ≤3.9 mg/dL and greater rates when SU ≥10 mg/dL, compared with SU 4.0–5.9 mg/dL (p for trend <0.01).

Conclusion Gout flare rates were persistently higher when SU ≥6 mg/dL after the first year of ULT after accounting for censoring. The spike in flares in all categories after stopping prophylaxis suggests a longer duration of prophylaxis may be warranted.