Diagnosis

If your doctor suspects that you have dermatomyositis, he or she might suggest some of the following tests:

• Blood analysis. A blood test will let your doctor know if you have elevated levels of muscle enzymes that can indicate muscle damage. A blood test can also detect autoantibodies associated with different symptoms of dermatomyositis, which can help in determining the best medication and treatment.
• Chest X-ray. This simple test can check for signs of the type of lung damage that sometimes occurs with dermatomyositis.
• Electromyography. A doctor with specialized training inserts a thin needle electrode through the skin into the muscle to be tested. Electrical activity is measured as you relax or tighten the muscle, and changes in the pattern of electrical activity can confirm a muscle disease. The doctor can determine which muscles are affected.
• MRI. A scanner creates cross-sectional images of your muscles from data generated by a powerful magnetic field and radio waves. Unlike a muscle biopsy, an MRI can assess inflammation over a large area of muscle.
• Skin or muscle biopsy. A small piece of skin or muscle is removed for laboratory analysis. A skin sample can help confirm the diagnosis of dermatomyositis. A muscle biopsy might reveal inflammation in your muscles or other problems, such as damage or infection. If the skin biopsy confirms the diagnosis, a muscle biopsy might not be necessary.