C3 and systemic lupus erythematosus: Positive predictive value (PPV), negative predictive value (NPV), and odds ratio (OR) for SLE diagnosis were 88%, 84%, and 37.667 for anti-dsDNA IgG class, 92%, 69%, and 23.731 for IgM, 89%, 65%, and 14.625 for IgA, 58%, 83%, and 10.679 for anti-C1q antibody, and 62%, 79%, and 6.097 for low C3 and/or C4 (Table 5).