Polymyositis is often suspected after a history and physical exam are done. Blood tests are commonly done to help make the diagnosis. When a person has this condition, the immune system often makes certain antibodies. These can be measured with a test called an antibody titer. Certain muscle proteins often increase in the blood due to muscle damage. For example, creatine phosphokinase and aldolase levels commonly rise. Another blood test, called an erythrocyte sedimentation rate, is also usually abnormal.
When these blood tests are abnormal, two other tests are commonly done to confirm the diagnosis: