How is dx of myositis done?

HX, PE, Labs (CPK, myoglobin, aldolase), Biopsy, Electromyography (motor unit potentials), Muscle MRI (gold std for study detailed view of muscle involvement, symmetrical edema T2 images and fatty atrophy T1 images) Biopsy (important to confirm inflammatory and muscle fiber changes, distinguish PM from IBM, exclude other myopathies, PM>DM, negative biopsy doesnot exclude myositis may be focal, Mononuclear infiltrates and muscle fiber necrosis common with all myositis. PM (MHC I cells invade), DM (perifascicular atrophy and microangiopathy), Immune mediated necrotizing myopathy (random necrosis w/out mononuclear infiltrate), Sporadic inclusion body myositis (endomysial inflammation, rimmed vacuoles and muscle fiber degeneration)