C4A and systemic lupus erythematosus: The sensitivity, specificity, positive and negative predictive values (PPV and NPV), and odds ratio (OR) (95% CI) for identification of patients with active SLE were 76.5%, 90.9%, 90.7% and 76.9%, and 14.625 (5.365–39.846) for a combination of anti-C1q and anti-dsDNA antibodies, 78.4%, 95.5%, 95.2% and 79.2%, and 23.030 (7.749–68.451) for a combination of anti-C1q antibody and levels of C3 and C4, and 72.5%, 93.2%, 92.5% and 74.5%, and 26.429 (7.987–87.549) for a combination of anti-C1q antibody, anti-dsDNA antibody, and levels of C3 and C4 anti-C1q Ab (Table 6).