C3 and systemic lupus erythematosus: We analyzed the serum complement C3 and complement C4 levels in 97 SLE patients with infection and in 59 SLE patients without infection and found that the levels of complement C3 in SLE patients with infection were similar to those in patients without infection (mean rank, 77.82 vs 79.61, U=2796.0, P=0.811; Figure 3A).