FRI0346 PERFORMANCE OF THE 2017 EUROPEAN LEAGUE AGAINST RHEUMATISM/AMERICAN COLLEGE OF RHEUMATOLOGY (EULAR/ACR) CLASSIFICATION CRITERIA FOR ADULT IDIOPATHIC INFLAMMATORY MYOPATHIES IN A HONG KONG COHORT

Background The new classification criteria for idiopathic inflammatory myopathy (IIM) endorsed by the European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) were published in 2017 [1]. However, the majority of the patients in the development cohort were Caucasians. External validation in other populations were advised. With the increasing availability and experience of myositis specific autoantibodies (MSAs) testing, the inclusion only anti-Jo-1 antibody is apparently insufficient. Objectives The objective of the study was to evaluate the performance of the 2017 EULAR/ACR classification criteria in a cohort of Hong Kong adult IIM patients. The secondary objectives included examining the level of agreement between the new criteria and the traditional criteria and assessing the effect of including other MSAs into the criteria. Methods This was a multi-centre retrospective cross-sectional study. Consecutive patients with a clinical diagnosis of IIMs and MSA tested seen in the rheumatology clinic or admitted to the rheumatology wards of the participating hospitals in Hong Kong up till August 2018 were recruited. Patients with juvenile onset myositis were excluded. Clinical parameters required by the two criteria will be collected by reviewing the medical records. A commercial line blot immunoassay kit (EUROIMMUN) was used to detect the MSAs. Results Two hundred and four patients with IIM were recruited. The mean age was 59.3 years. There was a female predominance of 76.5%. The subgroups of the patients were: polymyositis 40.7%, dermatomyositis 38.2%, clinically amyopathic dermatomyositis patients 21.1%. MSAs were detected in 59.3% of the patients with anti-Jo-1 antibody being the commonest (13.2%). The new 2017 EULAR/ACR Criteria could classify 96.1% of the patients as having definite or probable IIM. The Bohan and Peter criteria could only classify 76.0% of the patients. When combining with the Sontheimer’s criteria for CADM, 93.1% of the patients could be classified. The percentage agreement of the new and the Bohan and Peter criteria also increased from 77.0% to 93.1% when the latter was supplemented by the Sotheimer’s criteria. If the presence of any MSAs is considered one of the criteria, the performance of the 2017 criteria improved to 97.5% while the combined Bohan and Peter/Sontheimer’s criteria to 96.1%. However, the new criteria still failed to highlight the important subtype of IIM associated with anti-MDA5 autoantibody. Conclusion In a population with a significant proportion of CADM patients, the new 2017 EULAR/ACR classification criteria outperformed the old criteria. Finally, a clinico-serological criteria for “anti-MDA5 syndrome” is proposed in which a patient must have positive serologic testing for an anti-MDA5 autoantibody, plus one of the following conditions: myositis by the new EULAR/ACR criteria, interstitial lung disease or typical rash (skin ulceration, palmer papules). Reference: [1] Lundberg IE, Tjarnlund A, Bottai M, Werth VP, Pilkington C, Visser M, et al. European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Ann Rheum Dis. 2017;76:1955–64. Disclosure of Interests None declared