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Objective: Interstitial lung disease (ILD) in systemic sclerosis (SSc) runs a highly variable course, and prediction tools are highly desired. The aim of this study was to assess the diagnostic and prognostic performance of 4 candidate serum biomarkers for SSc-associated ILD., Methods: Serum samples from a combined cohort of SSc patients (from Paris, France and Oslo, Norway; n = 427) were analyzed by enzyme-linked immunosorbent assay for concentrations of lung epithelial-derived surfactant protein D (SP-D), Krebs von den Lungen 6 glycoprotein (KL-6), CCL18, and OX40 ligand (OX40L). Lung fibrosis was measured by high-resolution computed tomography and pulmonary function tests. Associations of these candidate biomarkers with baseline disease involvement and prediction of disease progression over time (mean ± SD follow-up 3.2 ± 4.4 years) were investigated., Results: In SSc patients at baseline, serum levels of KL-6 correlated with the forced vital capacity (FVC) (r = -0.317, P < 0.001), diffusing capacity for carbon monoxide (r = -0.335, P < 0.001), and extent of lung fibrosis (r = 0.551, P < 0.001). In multivariate analyses, serum levels of KL-6 and SP-D, but not CCL18 and OX40L, were associated with lung fibrosis (odds ratio [OR] 2.41, 95% confidence interval [95% CI] 1.43-4.07 [P = 0.001] and OR 3.15, 95% CI 1.81-5.48 [P < 0.001], respectively). In SSc patients with ILD at baseline, longitudinal, multivariate analyses showed that CCL18 serum levels were an independent predictor of a >10% decrease in the FVC (hazard ratio [HR] 2.90, 95% CI 1.25-6.73; P = 0.014) and de novo development of extensive disease (HR 3.71, 95% CI 1.02-13.52; P = 0.048). Matrix-based logistic regression models for the diagnosis and prognosis of SSc-associated ILD were constructed, and these models discriminated 3 groups of risk (mild, moderate, or high) for the diagnosis or worsening of lung fibrosis according to the serum levels of SP-D (for diagnosis) and serum levels of CCL18 (for progression of disease)., Conclusion: These results show that SP-D is a relevant diagnostic biomarker for SSc-associated ILD, whereas KL-6 could be used to assess the severity of lung fibrosis. CCL18 appears to be a potential predictive marker for progression of ILD in SSc., (© 2019, American College of Rheumatology.)