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Background The recently published 2017 European League Against Rheumatism (EULAR) and American College of Rheumatology (ACR) classification criteria for adult and juvenile idiopathic inflammatory myopathies (IIM) and their major subgroups reflect a long-appreciated need for more accurate case definition in ongoing research in these complex and heterogenous diseases.1 However a number of issues remain unresolved. There was a high frequency of missing data in both the derivation and validation samples, only one of the now numerous myositis specific autoantibodies is included, and certain well recognised IIM subtypes are not specifically classified despite their well phenotyped and differing natural histories, clinical features and treatment modalities.2 3 Objectives To perform an external validation of the EULAR/ACR classification criteria in an incident IIM cohort and examine how classification criteria-assigned IIM subtype correlates with expert opinion. Methods Adults with newly diagnosed IIM attending Salford Royal NHS Foundation Trust Neuromuscular services were identified. A retrospective review of all putative cases was performed, and cases fulfilling a consensus expert-opinion diagnosis of definite IIM included. A broad range of clinical, serological and histological data were collected and each case assigned a single IIM subtype by expert opinion. The EULAR/ACR classification criteria were applied and sensitivity, specificity, positive and negative predictive value calculated, presented with 95% confidence intervals (CI). Results A total of 922 cases were screened with 255 expert opinion definite IIM identified. The sensitivity to diagnose an IIM was 99.6% (97.2–100) and 80.9% (76.0–85.8) for the classification criteria cut-points of ‘probable’ and ‘definite’ respectively. The sensitivity for ‘definite’ IIM improved to 90.2% (86.5–93.8) when biopsy data for 24/34 initially missed cases were excluded. In 94/255 cases the IIM subtype differed between expert opinion and classification criteria, most strikingly in the group subtyped ‘polymyositis’ using the EULAR/ACR criteria, where there was discrepancy in the majority (87/161). Conclusions The criteria performed with high sensitivity in identifying IIM in an external cohort of IIM patients. However, substantial disagreement exists in subtype assignment, especially resulting in a larger proportion of cases of ‘polymyositis’ with heterogeneous features, important to consider in the application of these criteria to subsequent research. References [1] Lundberg IE, Tjarnlund A, Bottai M, Werth VP, Pilkington C, Visser M De, et al. EULAR/ACR classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Ann Rheum Dis2017;76:1955–64. [2] Betteridge Z, McHugh N. Myositis-specific autoantibodies: An important tool to support diagnosis of myositis. J Intern Med2016;280:8–23. [3] Longo DL, Dalakas MC. Inflammatory muscle diseases. N Engl J Med2015;372:1734–47. Disclosure of Interest None declared

Background The idiopathic inflammatory myopathies (IIM) represent a rare and heterogeneous group of multisystem autoimmune diseases. The rarity of IIM has hampered research efforts, resulting in remarkably limited therapeutic evidence. The EuroMyositis Registry was created to pool resources and expertise across the international IIM research community. Objectives To describe the phenotypic characteristics of different IIM subtypes. Associations with malignancy, interstitial lung disease (ILD), cardiac involvement and dysphagia were assessed. Methods Pooled data from the EuroMyositis Registry (from Belgium, China, Czech Republic, Hungary, Italy, Mexico, Norway, Sweden, Switzerland, UK, Vietnam) were obtained. Associations were assessed using logistic regression and multinomial logistic regression with polymyositis (PM) as the reference group. Results Data regarding 3,196 patients were analysed. The UK was the largest contributor (n=1,307). The most common diagnoses were dermatomyositis (34%), PM (32%) and connective tissue disease (CTD)-overlap myositis (15%). In those with anti-synthetase syndrome (7%), 85% had muscle weakness, 86% ILD and 58% arthritis. Overall, 43% had a myositis specific antibody, most commonly anti-Jo1 autoantibodies (20%). Glucocorticoid usage was noted in 98%. Most commonly used disease modifying agents were methotrexate (71%) and azathioprine (50%). Malignancy occurred in 9% and was associated with a diagnosis of dermatomyositis (Relative Risk Ratio [RRR] 1.68, 95% CI 1.09–2.56, p=0.018). Cardiac involvement occurred in 9%, most commonly in those with CTD-overlap myositis (13%), and was associated with a higher Health Assessment Questionnaire disability index (1 versus 0.75, OR 1.40, 95% CI 1.03–1.91, p=0.031). Dysphagia occurred in 39% and was associated with a diagnosis of CTD-overlap myositis (RRR 2.25, 95% CI 1.61–3.15, p Conclusions This large international cohort demonstrates the heterogeneity of IIM and the burden of associated malignancy, ILD, cardiac and gastrointestinal involvement. The EuroMyositis Registry facilitates international collaborative research outputs, underlining the benefits of harmonised data collection methodology between centres. Acknowledgements This work was supported by researchers at the National Institute for Health Research (NIHR) Biomedical Research Unit. The views expressed are those of the authors and not necessarily those of the UK National Health Service, the NIHR or the UK Department of Health Disclosure of Interest J. Lilleker: None declared, J. Vencovsky: None declared, G. Wang: None declared, L. Wedderburn Grant/research support from: The UK JDM Cohort and Biomarker study is supported by grants from the NIHR and Myositis UK, L. Diederichsen: None declared, J. Schmidt: None declared, P. Jordan: None declared, O. Benveniste: None declared, M. G. Danieli: None declared, K. Dankό: None declared, N. T. P. Thuy: None declared, M. Vazquez-Del Mercado: None declared, O. Molberg: None declared, B. De Paepe: None declared, J. De Bleecker: None declared, B. Maurer Grant/research support from: AbbVie, Protagen, EMDO, Novartis, Roche, Actelion, N. Pipitone Speakers bureau: GRAPPA Workshop, Alfa-Wassermann, N. McHugh: None declared, Z. Betteridge: None declared, P. New: None declared, R. Cooper: None declared, W. Ollier: None declared, J. Lamb Grant/research support from: MedImmune, N. S. Krogh: None declared, I. Lundberg Grant/research support from: Astra-Zeneca, Bristol-Myers Squibb, Consultant for: Bristol-Myers Squibb, Idera, H. Chinoy Grant/research support from: MedImmune, Novartis