C3 and systemic sclerosis: SSc patients with active DUs had similar CH50 [67.19 U/mL (IQR 49.72–76.32 U/mL) vs. 78.24 U/mL (IQR 66.21–91.75 U/mL); p > 0.05], C2 [25.8 mg/L (IQR 24–32.8 mg/L) vs. 27.6 mg/L (24.1–32.1 mg/L); p > 0.05], C3 [1.14 g/L (IQR 1.05–1.23 g/L) vs. 1.1 g/L (IQR 0.97–1.24 g/L); p > 0.05], C4 [0.2 g/L (IQR 0.18–0.23 g/L) vs. 0.2 g/L (IQR 0.16–0.23 g/L); p > 0.05], ESR [22 mm/h (IQR 12–39 mm/h) vs. 19 mm/h (IQR 11–29 mm/h); p > 0.05] and CRP [2400 mcg/L (IQR 1000–5200 mcg/L) vs. 1600 mcg/L (IQR 900–2600 mcg/L); p > 0.05] than SSc patients without active DUs.