Improved performance of confirmatory assays for detecting dense fine speckled (DFS) 70 antibodies

The presence of monospecific dense fine speckled 70 (DFS70) pattern in indirect immunofluorescence assay (IFA) without concomitant systemic autoimmune related diseases (SARD)-associated antibodies could be an exclusion biomarker for SARD. Since interpretation of DFS pattern on IFA is subjective, an assay for confirming the presence of anti-DFS70 is required. We evaluated the performance of two commercial assays [fluorescence enzyme immunoassay (FEIA) and line immunoassay (LIA)] for detecting anti-DFS70. Sera with monospecific DFS (n=176) and without DFS (n=179) pattern from referred patients for ANA testing, in two independent laboratories and healthy donors, were investigated for anti-DFS70 by FEIA (Phadia EliA) and LIA (Euroimmun). The assay performance including sensitivity and specificity at different cut-offs was evaluated, and optimal cut-offs were determined. The newly established optimal cut-offs (2.7 U/mL on EliA, band intensity of 28 on LIA) showed significantly better assay performance in detecting anti-DFS70 and confirming monospecific DFS pattern. A relative sensitivity of 93.7% with relative specificity of 100% on EliA and a relative sensitivity of 96.6% with relative specificity of 95.3% on LIA were achieved. Superior Cohen's Kappa agreements with IFA were also achieved for both assays (0.936 for EliA and 0.906 for LIA). Application of an optimal cut-off can significantly increase the assay performance for anti-DFS70 and enhance the accuracy in reporting DFS pattern by IFA.