Integrated interpretation improves the diagnostic performance of myositis line blot testing in fibrosing interstitial lung disease

Interstitial lung diseases (ILDs) encompass a wide range of etiologies, including idiopathic inflammatory myopathies (IIMs). Myositis-specific and myositis-associated autoantibodies (MSAs, MAAs) play a key role in the etiological diagnosis of ILDs. We evaluated the contribution of MSA and MAA testing by myositis line blot in the context of the initial workup of fibrosing ILDs. We conducted a retrospective study based on line blots results prescribed at the Lille University Hospital by the pulmonology services as part of the initial workup for fibrosing ILDs. Among the 142 patients included, 10 cases of IIM-associated ILD were identified (7%) and constituted the IIM-ILD group. Myalgia was more frequently reported in the IIM-ILD group compared with the non IIM-ILD group, in which ILD was attributed to an etiology unrelated to IIM (20.0%, 2/10 versus 4.6%, 6/132). In 3 patients of the IIM-ILD group, ILD was the only clinical manifestation. All patients in the IIM-ILD group had anti-synthetase antibodies, most commonly anti-EJ. Among non IIM-ILD group patients with a positive line blot, the most frequently identified reactivities were anti-Mi2β (n = 7), anti-PM/Scl (n = 7), anti-SRP (n = 5) and anti-Ku (n = 4). Expert interpretation of line blot results, integrating the characteristics of each detected autoantibody and HEp-2 indirect immunofluorescence (IIF) findings, yielded a high positive predictive value (87.5%) and negative predictive value (97.8%) while limiting false positives (specificity 99.2%). In conclusion, IIMs represent a non-negligible proportion of ILD etiologies, justifying systematic screening. Combining HEp-2 IIF and line blot improves diagnostic performance, provided that results are interpreted conjointly by the biologist.