C3 and systemic lupus erythematosus: C3 and C4 levels were more frequently reduced in SA- patients with SLE (low C3 levels: 19 patients (29.7 %) SA- vs 3 patients SA+ (16.6 %); P = 0.01; low C4 levels: 21 patients (32.8 %) SA- vs 4 patients SA+ (22.2 %; P = 0.03).