Improved Detection of Myositis‐Specific Autoantibodies Using Luciferase Immunoprecipitation Systems Assay: Comparison With Line Blot and Conventional Immunoprecipitation

OBJECTIVE: Myositis-specific autoantibodies guide the diagnosis and classification of idiopathic inflammatory myopathies, but current immunoassays vary in accuracy, particularly for autoantibodies associated with interstitial lung disease. To compare the performance of the luciferase immunoprecipitation systems (LIPS) assay with line blot and immunoprecipitation (IP) and to evaluate whether antibody-specific thresholds modify line blot concordance. METHODS: We analyzed 279 sera with line blot results from the Johns Hopkins Myositis Center. LIPS was performed for anti-Jo-1, anti-PL-7, anti-PL-12, anti-MDA-5, anti-EJ, and anti-Ro52 autoantibodies. Line blot results were evaluated using standard thresholds (≥36 and ≥71 arbitrary units [AU]). Assay performance was evaluated against IP (n = 141). In patients without IP (n = 138), clinical diagnoses were examined. RESULTS: LIPS showed excellent agreement with IP (κ = 0.90, area under the receiver operating characteristic curve 0.935), outperforming line blot. Line blot cutoffs influenced performance: ≥36 AU increased sensitivity (92.9%) but reduced specificity (73.8%), whereas ≥71 AU improved specificity (92.9%) but lowered sensitivity (82.8%). Antibody-specific thresholds modified concordance but remained quantitatively inferior to LIPS. In patients without IP, concordance between LIPS and line blot was 89% at ≥36 AU and 83% at ≥71 AU. LIPS-positive cases were enriched for antisynthetase syndrome and dermatomyositis. Among discordant cases (line blot positive and LIPS negative), most were clinically classified as inclusion body myositis (5 of 11). For anti-Ro52, discrepancies were most frequent at titers