OBJECTIVE:
The objective was to investigate blood-based biomarkers of type I (PRO-C1), III (PRO-C3) and VI (PRO-C6) collagen formation in systemic sclerosis (SSc) patients and examine their correlation to modified Rodnan skin score (mRSS).

METHODS:
Limited (lSSc, n = 76) and diffuse SSc (dSSc, n = 41) fulfilling the ACR/EULAR 1980 and 2013 classification criteria for SSc and asymptomatic controls (n = 9) were included. PRO-C1, PRO-C3 and PRO-C6 were measured in serum.

RESULTS:
LSSc compared to dSSc were significantly older, had longer disease duration and lower mRSS. PRO-C3 was higher in early dSSc compared to early lSSc (mean [95 percentile], 27.4 [13.1-39.1] ng/mL vs 14.9 [8.2-28.8] ng/mL, p = 0.006). PRO-C6 levels were higher in early dSSc compared to early lSSc and late dSSc (early dSSc: 28.2 [10.4-92.3] ng/ml vs early lSSc: 11.0 [6.9-28.5] ng/ml; p = 0.006 and late dSSc: 12.6 [6.5-25.3] ng/mL, p = 0.04). No difference was observed with PRO-C1. PRO-C3 and PRO-C6 were moderately correlated with mRSS with R-partials of 0.36 (p < 0.001) and 0.29 (p = 0.002), respectively Conclusion: Measures of type III and VI collagen formation are potential objective biomarkers of fibrosis in systemic sclerosis. These biomarkers could be useful in monitoring the disease and efficacy of treatment.