Assay performance characteristics of the anti-SmD3 peptide (SMP) assay. (a) Intra-assay and interassay variability, (b) linearity, and (c) receiver operating characteristic analysis. The intra-assay and interassay variability, expressed as coefficient of variation in percentage (CV%), of three samples ranged from 1.82 to 6.52% and from 2.27 to 7.42%, respectively. Serial dilution series of two samples with high titres of anti-Sm antibodies (S6 and H) exhibited a linear binding response (<20% deviation). Definition of the cutoff, using receiver operating characteristic (ROC) analysis, was performed with SLE and control sera.