Assessment of disease activity in systemic sclerosis (SSc) is limited by the absence of a fully validated, multisystem measure of disease activity. The European Scleroderma Trials and Research Group (EUSTAR) SSc activity index (EScSG-AI) was recently revised, and a validation study within the EUSTAR cohort was performed. In this study, we evaluated the performance of the revised EScSG-AI in an external Australian cohort. The association between the EScSG-AI and the physician global assessment of disease activity (PhGA), both collected prospectively at each annual visit over up to 12 years follow-up, was evaluated using Pearson's correlation coefficient and Cohen's kappa coefficient. Generalized linear modelling and time-dependent Cox regression analysis were performed to determine the association of disease activity measured by the EScSG-AI and the summed Medsger Severity Scale (MSS) and death, respectively. There was a moderate correlation between EScSG-AI and PhGA scores (r 0.42, p < 0.001) and moderate association between rising EScSG-AI and summed MSS (r 0.60, p < 0.001). High disease activity, measured by the EScSG-AI at any time during follow-up, was associated with a hazard ratio of 2.07 (95% CI 1.51-2.79) for mortality. The EScSG-AI has a moderate correlation with physician-assessed SSc disease activity. This suggests that the criterion and construct validity of the EScSG-AI are yet to be demonstrated in an external cohort of SSc patients. Key Points •There remains no gold standard measure of SSc disease activity. •The revised 2017 EUSTAR SSc disease activity index shows moderate correlation with physician-rated global disease activity. •Significant work remains to develop a validated multisystem measure of disease activity in SSc.
Keywords: Disease activity; Outcome assessment; Systemic sclerosis; Treatment outcome.