Minimally important differences of the gout impact scale in a randomized controlled trial

Abstract

Objective: The Gout Impact Scale (GIS) is a gout-specific quality of life instrument that assesses impact of gout during an attack and impact of overall gout. The GIS has five scales and each is scored from 0 to 100 (worse health). Our objective was to assess minimally important differences (MIDs) for the GIS administered in a randomized controlled trial (RCT) assessing rilonacept vs placebo for prevention of gout flares during initiation of allopurinol therapy.

Methods: Trial subjects (n = 83) included those with two or more gout flares (self-reported) in the past year. Of these, 73 had data for Weeks 8 vs 4 and formed the MID analysis group and were analysed irrespective of the treatment assignment. Subjects completed the GIS and seven patient-reported anchors. Subjects with a one-step change (e.g. from very poor to poor) were considered as the MID group for each anchor. The mean change in GIS scores and effect size (ES) was calculated for each anchor's MID group. The average of these created the overall summary MID statistics for each GIS. An ES of 0.2-0.5 was considered to represent MID estimates. Results. Trial subjects (n = 73) were males (96.0%), White (90.4%), with mean age of 50.5 years and serum uric acid of 9.0 mg/dl. The mean change score for the MID improvement group for scales ranged from -5.24 to -7.61 (0-100 scale). The ES for the MID improvement group for the four scales ranged from 0.22 to 0.38.

Conclusion: The MID estimates for GIS scales are between 5 and 8 points (0-100 scale). This information can aid in interpreting the GIS results in future gout RCTs. Trial Registration. Clinicaltrials.gov, www.clinicaltrials.gov, NCT00610363.