OA12 Autoantibodies are common in patients labelled as “idiopathic” interstitial lung disease suggesting a high prevalence of undiagnosed autoimmune connective tissue disease

Abstract Background/Aims In some patients, interstitial lung disease (ILD) may be the dominant or even sole clinically overt manifestation of an otherwise unrecognised autoimmune connective tissue disease (CTD). Accurate diagnosis can be challenging given considerable overlap of the clinical, radiological and histological disease features. Distinguishing CTD-related ILD (CTD-ILD) from idiopathic ILD can avoid a lung biopsy, enables appropriate immunosuppressive treatment, informs prognosis and facilitates monitoring for other CTD-associated complications. Autoantibodies are a hallmark feature of CTD, are highly disease-specific and their presence is strongly suggestive of covert CTD-ILD. We investigated patients who had been labelled with “idiopathic” ILD by their local hospital teams for the presence of autoantibodies that may suggest misdiagnosis. Methods Serum from two groups of patients recruited to UK Biomarkers of Interstitial Lung Disease (BILD) were analysed: 183 with a local diagnosis of idiopathic non-specific interstitial pneumonia (iNSIP) and 34 with a local diagnosis of idiopathic or Cryptogenic Organising Pneumonia (COP). Autoantibody status was determined by radio-immunoprecipitation. Results Overall, CTD-autoantibodies were identified in 15% of patients with a prior label of “idiopathic” ILD. CTD autoantibodies were more common in those with a prior diagnosis of COP, where more than 1 in 5 patient samples contained a potentially relevant autoantibody, see table. The autoantibodies identified included those readily detectable e.g. anti-Jo1, in addition to rarer antibodies not included in standard assays e.g. anti-EIF2B. Nearly half of all autoantibodies detected were anti-synthetase autoantibodies. Conclusion Covert CTD-ILD is potentially being missed as a diagnosis in patients who have been labelled by local teams as having idiopathic forms of inflammatory ILD (COP or iNSIP). More specific guidance on autoantibody testing and the interpretation of these test results could improve diagnosis and ensure patients receive more appropriate management. Disclosure S.L. Tansley: None. C.V. Cotton: None. F.K. McMorrow: None. H. Lu: None. Z.E. Betteridge: None. R.P. New: None. L.G. Spencer: None. N.J. McHugh: None. R.G. Cooper: None.