C4A and systemic lupus erythematosus: Martin et al. found that high C4d can be a potential marker for discriminating LN from non-renal SLE with an OR of 0.8 and an AUC of 0.71; moreover, the ratio of C4d to C4 seems to be a better predictor for LN with an OR of 13.1, AUC 0.76, and a Sn of 83% with a Sp of 73% [111].