Your doctor will ask you to describe your symptoms. It is especially important to describe the exact location of any weakness or muscle pain and how long you have had it. The doctor also will review your medical history and current medications.
Next, your doctor will examine you, paying special attention to your muscles and nerves. Depending on what your doctor finds, he or she may order diagnostic tests. Common tests used to diagnose muscle problems include:
Blood tests to measure levels of muscle enzymes, auto-antibodies (antibodies directed against one's own cells or organs) and antibodies to infectious agents
An electromyogram, a test that measures the electrical activity of muscles
Magnetic resonance imaging (MRI), a painless scan that can identify abnormal muscle and can be used to locate the best site for a biopsy to establish the diagnosis or to monitor the progress of a known type of myositis
A standard X-ray or a bone scan if your doctor suspects you have myositis ossificans
A computed tomography (CT) scan or MRI if pyomyositis is suspected
A muscle biopsy. With this test, the doctor takes a sample of muscle tissue to be checked under the microscope. Muscle biopsy is the best way to establish the diagnosis of myositis; it may be diagnostic if it shows clear muscle inflammation. However, the results are not always abnormal even in people who have myositis.