Abstract: Objective To investigate the classification of idiopathic inflammatory myopathies (IIM) based on clinical manifestations and myositis- specific antibodies using cluster analysis. Methods We retrospectively analyzed the data of patients with IIM admitted in Nanfang Hospital in 2015-2019. The clinical data of the patients including serum creatine kinase (CK), interstitial lung disease (ILD), cancer, and myositis-specific antibodies were collected for two-step cluster analysis to identify the distinct clusters of patients, whose clinical characteristics were subsequently analysed. Results A total of 71 patients with IIM were included in this study, including 30 (42.3% ) with polymyositis (PM), 20 (28.2% ) with classic dermatomyositis (DM), 16 (22.5% ) with amyopathic dermatomyositis (CADM), and 5 (7.0% ) with immune-mediated necrotizing myopathy (IMNM). Two-step cluster analysis identified 3 distinctive subgroups: Cluster 1 of 15 (51.7%) patients characterized by rash, positive anti-MDA5 antibody and hypoproteinemia (P<0.05) with normal or slightly elevated CK level, mainly corresponding to CADM; Cluster 2 of 4 (57.1%) patients with significantly elevated CK and positive anti-SRP antibody (P<0.001) corresponding to IMNM; and Cluster 3 of 17 (48.6%) patients consisting primarily of patients with PM, characterized by positivity for anti- aminoacyl transfer RNA synthetases antibodies (P=0.022) corresponding to antisynthetase syndrome (ASS). Conclusions Patients with IIM can be divided into 3 subgroups based on their clinical and serological characteristics (especially myositis-specific antibodies), and among them ASS may represent an independent IIM subgroup with unique clinical characteristics.

Key words: idiopathic inflammatory myopathies, amyopathic dermatomyositis, immune-mediated necrotizing myopathy; myositis specific antibody, two-step cluster analysis