Likewise, GPA patients displayed significantly higher C4d levels compared to MPA patients [0.50 (0.40–0.71) mg/L vs 0.38 (0.27–0.47), p = 0.029, Fig. 2e], but there was no difference in C4d/C4 ratio (Fig. 2g). Here, C4A is linked to granulomatosis with polyangiitis.