C4A and polymyositis: DM patients have higher incidence of interstitial lung disease (ILD) than PM patients and PM patients had higher levels of lactic dehydrogenase (LDH) than DM patients, but there was no difference in creatine kinase (CK), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), immunoglobulin A (IgA), IgM, and IgG, component 3 (C3) and C4, anti-Jo-1 autoantibody, and anti-Ro-52 autoantibody.