C4A and systemic lupus erythematosus: Further, PC4d correlated with C3dg, a circulating marker of complement activation (measured in 209 of the SLE patients, rs = 0.46, P < 0.0001; Fig. 1B), and negatively with levels of C3 and C4 (rs = –0.28, P < 0.0001, and rs = –0.35, P < 0.0001, respectively).