Analytic and Clinical Validity of Myositis-Specific Antibodies by Line-Blot Immunoassay Is Essential.

OBJECTIVES: This study assessed the concordance between line blot (LB) and immunoprecipitation (IP) assays for detecting myositis-specific antibodies (MSAs) in idiopathic inflammatory myopathies (IIMs) and their association with IIM subtypes.

METHODS: One hundred patients with IIM were enrolled, and MSA was detected using LB and IP. The IIM subtypes, including immune-mediated necrotizing myopathy-like, anti-tRNA synthetase syndrome-like, and clinically amyopathic dermatomyositis-like, were clinically diagnosed. The validity and reliability of the LB compared with the IP were evaluated. Optimal cutoff levels for LB were determined using various statistical methods including Cohen κ, Gwet's AC, diagnostic odds ratios, and receiver operating characteristic analysis.

RESULTS: Line blot exhibited lower specificity and accuracy than IP in predicting IIM subtypes. Some MSAs performed better at higher LB cutoff values. Anti-signal recognition particle antibodies showed poor performance in predicting the immune-mediated necrotizing myopathy-like subtype using LB. Raising the cutoffs improved the reliability of anti-threonyl-tRNA synthetase and anti-signal recognition particle antibodies. Anti-histidyl-tRNA synthetase antibodies performed well at lower positivity, whereas diagnostic odds ratios increased for anti-transcription intermediary factor 1γ and anti-nuclear matrix protein 2 with higher cutoffs.

CONCLUSIONS: Inconsistencies between LB and IP have been observed in patients with IIM. Individual optimal cutoffs for MSA by LB correlating with IP were determined. Rheumatologists should consider the differences between LB and IP results when classifying IIM subtypes.