Ultrasonographic measurement of tophi as an outcome measure for chronic gout.

OBJECTIVE: To validate the usefulness of measuring tophi with ultrasonography (US) as an outcome measure for chronic tophaceous gout.

METHODS: Patients with crystal-proven gout were included. To evaluate validity, intraarticular and articular deep tophi were evaluated with both magnetic resonance imaging (MRI) and US. Tophi were punctured with US guidance to evaluate face validity. Interobserver and intraobserver measurement studies were done to evaluate reliability, and to estimate the smallest detectable difference. Sensitivity to change was evaluated with a 12-month followup observational study of urate-lowering therapy.

RESULTS: US detected at least one tophus in all joints where MRI found nodules considered to be tophi. There was a good correlation, but just fair agreement between measurements with US and MRI. Puncture of nodules suspected of being tophi recovered urate crystals in 83% of the procedures. Intraobserver intraclass correlation was > 0.90 for diameters and volume, while it was 0.71 to 0.83 in the interobserver study. US was found to be sensitive to change, and there was an inverse correlation between serum urate concentrations and change from baseline measurement of tophi.

CONCLUSION: US measurement of tophi fulfilled the OMERACT filter for an outcome measure, although it should be tested further in randomized clinical trials.