[Evaluation of cut-off value for autoantibodies against double-stranded DNA-complexed nucleosomes based on enzyme...

The new classification criteria for systemic lupus erythematosus (SLE) by Systemic Lupus International Collaborating Clinics (SLICC) published in 2012 have defined positive level titer of antibody against double stranded (ds) DNA as more than double the reference range if tested by enzyme linked immunosorbent assay (ELISA). The aim of this study was to evaluate optimal cut-off value and diagnostic performance of the anti-dsDNA nucleosome-complexed (anti-dsDNA-NcX) ELISA, which uses salmon testis DNA complexed with purified nucleosomes as antigens, for diagnosis of SLE. Titers of antibodies against dsDNA-complexed nucleosome were measured in sera of 76 patients with SLE, 148 with other connective tissue diseases, and 323 healthy volunteers. Sensitivity, specificity, correlation and concordance rate were compared among anti-dsDNA-NcX, conventional ELISA methods (EIA) and radioimmunoassay (RIA). As a results, concordance rates and Spearman's coefficient of rank correlation (r(s)) of anti-dsDNA-NcX with EIA and RIA were 59.2%, r(s) = 0.40 and 53.9%, r(s) = 0.21, respectively. Receiver operating characteristic curve analysis showed that the titer of 44 IU/mL calculated as 99 percentile of 323 healthy volunteers is a better cut-off value for anti-dsDNA-NcX than the 100 IU/mL recommended by the manufacturer. By setting the cut-off value at 44 IU/mL, anti-dsDNA-NcX showed the highest sensitivity (75.0%) and specificity (90.5%) of the 3 assays. With SLICC criterion, positivity rates of anti-dsDNA-NcX, EIA and RIA for SLE patients were 50.0%, 30.3% and 50.0%, respectively. In conclusion, anti-dsDNA-NcX has a good diagnostic performance for SLE with our proposed cut-off value of 44 IU/mL.