Performance of the new EULAR/ACR classification criteria for idiopathic inflammatory myopathies (IIM) in a large monocentric IIM cohort.

Background: In 2017, the European League Against Rheumatism (EULAR) and the American College of Rheumatology (ACR) published new classification criteria for idiopathic inflammatory myopathies (IIM).

Objectives: To [1] assess the performance of the EULAR/ACR criteria in a monocentric cohort of consecutive patients with IIM, compare them with the Bohan and Peter (BP) criteria, and with the physician's diagnosis; and [2] evaluate the effect of including the presence of interstitial lung disease (ILD) as variable in the criteria.

Methods: 439 consecutive patients with a diagnosis of IIM followed at the Rheumatology Clinic, Karolinska University Hospital, Sweden were enrolled. The patients were diagnosed as IIM and subclassified by expert physicians. Clinical, laboratory, serological and histopathological data were collected from existing databases (Euromyositis registry and Swedish Rheumatology quality registry) and clinical charts of the patients. The sensitivity of the EULAR/ACR and the BP criteria was calculated.

Results: The EULAR/ACR criteria had a higher sensitivity (87.7%) compared to the BP criteria (80.4%). The concordance between the two sets of criteria was low (k = 0.253 p<0.001). The EULAR/ACR criteria showed a very high specificity (>98%) for the major IIM subgroups polymyositis, dermatomyositis, and inclusion body myositis. The sensitivity was variable and was high in inclusion body myositis (98%), dermatomyositis (90%) and lower in polymyositis (73%). When including ILD in the variables of the criteria, six more patients were classified as IIM cases (1.3%).

Conclusion: The EULAR/ACR criteria for IIM are applicable with high sensitivity and specificity using data available from existing databases and clinical charts and represent a major step forward from the previous criteria for IIM and its subgroups. Their application will improve the quality of clinical trials and research studies with IIM patients.