Evaluate for emergent complications (e.g. respiratory distress, cardiomyopathy, arrhythmia)
BMP
ECG, troponin to screen for  myocardial involvement
CK, Urinalysis for myoglobin
Signs of systemic inflammation
ESR elevated in 50% of patients, does not correlate with disease activity
Elevated CRP
ESR elevated in 50% of patients, does not correlate with disease activity
Elevated CRP
ANA, Anti-Jo-1
Bedside PFTs (FEV1 and FVC)
EMG or muscle biopsy showing myositis
Malignancy screening/evaluation
Increased risk of malignancy (~30%), especially: ovarian, lung, pancreatic, stomach, colorectal, bladder, and non-hodgkin lymphoma
Associated with increased age, male gender and elevated creatinine kinase levels
Increased risk of malignancy (~30%), especially: ovarian, lung, pancreatic, stomach, colorectal, bladder, and non-hodgkin lymphoma
Associated with increased age, male gender and elevated creatinine kinase levels
Dermatomyositis
Infection:
Polymyositis
Viral infection 
Pyomyositis
Bacterial infection
Lambert-Eaton myasthenic syndrome
Spirochete infection 