Performance of cut-offs adjusted with positive control band intensity in line-blot assays for myositis-specific antibodies

The diagnostic performance of band intensity (BI) cut-offs, adjusted by a positive control band (PCB) in a line-blot assay (LBA) for myositis-related autoantibodies (MRAs) is investigated. Sera from 153 idiopathic inflammatory myositis (IIM) patients with available immunoprecipitation assay (IPA) data and 79 healthy controls were tested using the EUROLINE panel. Strips were evaluated for BI using the EUROLineScan software, and the coefficient of variation (CV) was calculated. Sensitivity and specificity, area under the curve (AUC), and the Youden's index (YI) were estimated at non-adjusted or PCB-adjusted cut-off values. Kappa statistics were calculated for IPA and LBA. Although inter-assay CV for PCB BI was 3.9%, CV was 12.9% in all samples, and a significant correlation was found between BIs of PCB and seven MRAs (all P < 0.05). At adjusted BI (aBI) > 10, the negative conversion rate of myositis-specific autoantibody (MSA)-positivity at BI > 10 was 11.5% in controls and 1.3% in patients. The specificity, AUC, and YI for MSAs at aBI > 10 or > 20 were higher than those at non-adjusted cut-off values. Additionally, AUC (0.720), YI (0.440), and the prevalence of MRAs with kappa > 0.60 (58.3%) were the highest at aBI > 20. The overall sensitivity and specificity for MSAs were 50.3% and 93.7% at aBI > 20, respectively, and 59.5% and 65.8% with BI > 10, respectively. The diagnostic performance of LBA can be improved using PCB-adjusted BIs. aBI > 20 is the optimal cut-off for IIM diagnosis using the EUROLINE LBA panel.