C3 and diabetic kidney disease: At 1-year follow-up, exploratory longitudinal analysis showed that patients with lower C3 (<1.10 g/L) showed significantly greater 24-h proteinuria progression (-2869.27 vs. 250.46 mg/24h, <i>P</i> = 0.040).<h4>Conclusion</h4>In DKD patients with eGFR <90 ml/min/1.73m2, reduced serum C3 levels are associated with specific renal pathological injuries and may serve as a biomarker of disease progression.