Are serum C3 levels or kidney C3 deposits useful markers for predicting outcomes in patients with ANCA-associated vasculitis?

Complement activation emerged as a key actor of anti-neutrophil cytoplasmic antibodies-associated vasculitis (AAV). Whether serum levels of C3 (sC3) or C3 kidney deposition may help to refine the prognosis of AAV remains elusive. Retrospective multicentric study that included 154 patients with a first flare of AAV and sC3 (n = 143) or C3 kidney staining (n = 95) available at diagnosis. Clinical presentations, kidney pathology, and survival of patients with normal or low sC3 were compared using univariate analyses, Kaplan-Maier curves with log-rank comparison, or multivariate Cox’ model, as appropriate. 20 patients (14 %) had low sC3. sC3 (as bivariate low/normal or as a continuous variable) was associated with 5-year mortality but not with kidney survival. C3 kidney deposition (C3+) was identified in 23 patients who were characterized by more frequent chronic hypertension and lower eGFR at presentation (p = 0.04). C3+ correlated with IgG, IgM, C1q deposition (p = 0.07, p