Background Disease activity is a challenging concept in gout. Single measures are still preferred based on a priori considerations and preliminary evidence . Nevertheless the concept of disease activity in gout encompasses different interrelated aspects, including urate deposits, acute symptoms and disease progression, which should be taken into account together in driving treatment decisions.
Objectives To develop a new composite disease activity score for gout and provide preliminary validation.
Methods Disease activity has been operatively defined as presence of urate deposits that lead to acute arthritis and joint damage. Every item of the pool of measures for each core domain recommended by OMERACT for trials in gout  was tested for suitability in defining the concept of disease activity through a Delphi exercise. A three-step approach, similar to the methodology used for the disease activity score in rheumatoid arthritis, was applied to derive the gout activity score (GAS) . Decision to change treatment was used as surrogate criterion of high disease activity. Baseline and 12-month follow-up data of 446 patients included in the KING cohort (Kick-off of the Italian Network for Gout) were used. Data included measures of all domains identified as important for trials in gout. Construct and criterion-related validity were preliminary tested using risk of flare and cross-sectional and longitudinal health assessment questionnaire score (HAQ).
Results Factor analysis carried on variables selected by Delphi identified 5 factors: patient-reported outcomes, joint examination, attacks, tophi and serum uric acid (sUA). Discriminant function analysis using loadings of the 5 derived factors as independent variables and change of treatment as grouping variable resulted in a correct classification of 74% of cases. Linear regression analysis using the discriminant score as dependent variable and transformation of the original measures as independent variables resulted in a first candidate composite score (figure) including: number of last 3 months attacks, sUA, visual analogic scale (VAS) of pain (0-10), VAS global activity assessment (0-10), swollen and tender joint count (SWJ and TJC) and cumulative measure of tophi (cm). Alternative scores were also developed. GAS was significantly associated with the 6-month risk of flare (odds ratio 1.59 [95% CI 1.27, 1.98]) and to HAQ deterioration (beta coefficient -0.12 [95%CI -0.16, -0.07]). Temporal validation of the GAS showed consistent results.
Conclusions This study represents the first step in the development of a composite measure of disease activity in gout. Further testing is required in order to increase feasibility and to explore external validity and responsiveness of this instrument.
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Disclosure of Interest None Declared