Objective Domains identified as a result of qualitative research and Delphi exercises to assess rheumatoid arthritis (RA) flare include pain, function, swollen and tender joints, patient and physician global, laboratory measures, participation, stiffness, self-management and fatigue. Here we examine aspects of construct and content validity of these domains in a longitudinal observational study.

Methods A total of 1195 patients with RA treated with non-biological disease-modifying antirheumatic drugs (DMARDs) or biologics were eligible for the analyses. Working definitions of ‘flare’ included patient-reported worsening between 3 and 6 months (primary) and treatment change at 6 months (DMARDs and/or systemic corticosteroids) (secondary). Available outcome measures were mapped to the flare domains. Changes between 3 and 6 months were compared between patients with and without ‘flare’. Convergent and divergent construct validity and content validity were assessed by correlation analyses and logistic regression analysis, respectively.

Results Applying the flare working definition based on patient-reported worsening, standardised mean differences (SMDs) were >0.5 for the majority of outcomes. The largest SMDs were observed for Pain visual analogue scale (1.30), SF-36 Bodily pain (1.24), Patient global (1.20) and morning stiffness intensity (1.17). The flare working definition based on treatment change yielded lower SMDs (<0.5 for most variables). Consistently stronger intradomain than corresponding interdomain correlations supported convergent and divergent validity of the domains.

Conclusions Probing a flare definition via outcome measures, the identified flare domains discriminated well between patients with and without worsening. Interdomain and intradomain correlation and logistic regression analyses provide further support for construct and content validity of the identified flare domains.