As with other muscle diseases, a doctor diagnoses polymyositis (PM) by considering an individual’s history, family medical history and the results of a careful physical examination. This may be followed by some lab tests, perhaps of the electrical activity inside the muscles and usually a muscle biopsy.

After a careful history and physical exam to document the pattern of weakness in muscles, a doctor who suspects myositis likely will order a blood test to check the level of creatine kinase (CK), an enzyme that leaks out of muscle fibers when the fibers are being damaged. In PM, the CK level is usually very high.

In some cases, the doctor may ask for a blood test for specific antibodies, proteins produced by the immune system in myositis and other autoimmune diseases. Some of these antibodies appear to be specific to autoimmune muscle disease.

The next step is sometimes an electromyogram, a test in which tiny needles are inserted into the muscles to test their electrical activity both at rest and when the person tries to contract the muscle.

Inflammatory myopathies show a distinctive pattern of electrical activity that can help differentiate them from other types of muscle disease.

A nerve conduction velocity test is sometimes performed. This test measures how fast a nerve impulse travels and how strong it is.

Sometimes these tests are used to rule out disorders that may mimic the symptoms of inflammatory myopathies.