C4A and systemic lupus erythematosus: Laboratory measures for this cohort of SLE patients with joint pain prior to the US examination included (medians and IQRs): anti-dsDNA (4.0 IU/mL, 4.0–12.0 IU/mL), C3 (131.0 mg/dL, 114.0–144.0 mg/dL), C4 (30.0 mg/dL, 19.0–37.0 mg/dL), ESR (23.5 mm/h, 11.8–49.5 mm/h), and CRP (3.8 mg/L, 1.3–10.3 mg/L).