Receiver operating characteristic (ROC) curves for the three antibodies quantified. (A) Anti-ribosomal P (anti-Rib-P), (B) anti-Smith (anti-Sm), and (C) anti-double-stranded DNA (anti-dsDNA) antibodies. The curves represent the sensitivity and specificity for the systemic lupus erythematosus (SLE) group versus the healthy control group, and for the SLE group versus the rheumatic disease control group. For each curve, the area under the curve (AUC) and corresponding P-value are indicated. The flag indicates the cut-off established by the manufacturer and the asterisk indicates the new cut-off calculated from the curves (4.45 U/ml for anti-Rib-P and 3.4 U/ml for anti-Sm antibodies; the anti-dsDNA cut-off did not change).