Background <p>The forced vital capacity (FVC) is the most commonly used endpoint in registrational trials for systemic sclerosis-associated interstitial lung disease (SSc-ILD). However, the FVC has known methodological pitfalls, is affected by extra-pulmonary SSc manifestations and may not be clinically meaningful to patients. Combining individual outcomes into a composite endpoint is an attractive alternative for measuring treatment response and augmenting statistical efficiency in SSc-ILD trials.</p> Methods <p>We previously developed a composite endpoint for SSc-ILD using data from the Scleroderma Lung Study (SLS) I (comparing cyclophosphamide versus placebo for SSc-ILD), which included physiological (FVC), radiological (quantitative extent of fibrosis in the zone of maximum involvement) and patient-reported outcomes (transitional dyspnea index and health assessment disability questionnaire), which demonstrated a more robust treatment effect of cyclophosphamide than the FVC alone. The purpose of this post-hoc analysis was to validate this composite endpoint in an external clinical trial cohort (SLS II [cyclophosphamide versus mycophenolate for SSc-ILD). A secondary goal was to determine whether the composite endpoint predicted long-term mortality in SLS I and II.</p> Results <p>Seventy-two of the 142 randomized participants in SLS II had all composite endpoint components at 24 months and were included in this analysis. In both SLS I and II, the standardized effect size (Cohen’s d) was greater when the composite endpoint model was applied than when the FVC alone model was applied. In SLS I and II, the composite endpoint was a better predictor of long-term survival (HR 0.76 vs. 0.98 and 0.59 vs. 0.98, for the composite index vs. FVC Cox proportional hazards models in SLS I and II, respectively). Patients with high composite outcomes scores in both SLS I and II had significantly worse long-term survival than patients with low scores (<i>p</i> = 0.039 for log-rank test).</p> Conclusion <p>The results of this post-hoc analysis provide further evidence that a composite endpoint comprised of physiological, radiological and patient-reported outcomes is a promising endpoint for SSc-ILD trials. Future validation studies are needed.</p> Trial registration <p>SLS II: NCT00883129 (Date of registration April 17, 2009); SLS I: NCT00004563 (Date of registration: February 10, 2000).</p>