C4A and systemic lupus erythematosus: ROC analysis distinguishing ANA-positive (ANA+) SLE (N = 91) from ARD, TIgG, BC4d and TC4d demonstrated AUC values 0.81, 0.80 and 0.79, respectively, outperforming anti-dsDNA (0.72), C3 (0.69), TIgM (0.67), C4 (0.66) and anti-Smith (0.61).