ObjectiveTo determine the serum MDA5 levels and their clinical associations in patients with polymyositis/dermatomyositis (PM/DM).MethodsSerum anti-MDA5 antibody was detected by ELISA in 119 adult PM/DM patients,30 patients with systemic lupus erythematosus(SLE),30 patients with rheumatoid arthritis (RA),15 patients with primary Sj(O)gren's syndrome (SS),21 patients with pulmonary infection and 50 healthy controls.t-test,Mann-Whitney U test or chi-square test or Fisher exact test as well as Logistic multivariate regression analysis were carried out to compare the results of this study.ResultsSerum antiMDA5 antibody positive rate in DM patients(22.6％) were significantly higher compared with that of patients with PM (0,P＜0.01),patients with SLE (3.3％,X2=5.68,P＜0.05),RA (3.3％,X2=5.68,P＜0.05),pSS (0,P＜0.05) and pulmonary infection(0,P＜0.05) and healthy controls (0,P＜0.01).In the DM subgroup,CADM patients presented a higher positive anti-MDA5 antibody rate than classic DM patients.The incidence of arthritis,fever,vrash raised CEA and CA153 level,and serum concentration of GGT and ferritin were significantly higher in the anti-MDA5 positive DM group than anti-MDA5 negative DM group (X2=4.08,8.06,6.357,32.4,4.867; Z=-2.86,-2.44; P value of all these tests were less than 0.05 ),while the rate of serum positive ANA,serum level of CK and T,NK cell counts in the peripheral blood were much lower than those in anti-MDA5 negative DM group(X2=4.08; Z=-2.072,-2.013,-2.907; all P＜0.05).Moreover,the incidence of acute/subacute interstitial pneumonia(A/SIP) was significantly higher in anti-MDA5 positive DM patients than anti-MDA5 negative DM patients.The sensitivity and specificity of anti-MDA5 antibody for diagnosing A/SIP in DM patients were 88.2％ and 94％ respectively.Additionally,logistic multivariate analysis showed that anti-MDA5 was an independent risk factor for death of interstitial lung diseases (ILD) in DM (OR=8.46,95％CI 1.77～40.36,P＜0.01).ConclusionIn Chinese PM/DM patients,serum anti-MDA5 antibody is mainly present in DM patients and is a strong predictor for poor prognosis diagnosis of DM with A/SIP and is an independent risk factor for death of ILD in DM.