Dermatomyositis can often be diagnosed based on assessment of symptoms. Although the disease primarily affects the muscles and skin, it is a systemic condition and can also involve the lungs, joints, esophagus and sometimes the heart.

Examples of the steps that may be taken in the diagnosis of dermatomyositis include:

• Details of the history of the condition are taken along with a description of the onset, progression and severity of symptoms. Details of any other autoimmune or connective tissue disorders are obtained. The details of any family history of these diseases are also obtained.
• A skin biopsy can confirm a diagnosis and also rule out other conditions.
• Blood tests may be performed to check the levels of muscle enzymes such as creatinine kinase and aldolase, which may be raised in this condition, indicating muscle damage.  Blood tests can also reveal the presence of specific antibodies that are associated with certain forms of dermatomyositis and therefore aid treatment decisions.
• Electromyography may be carried out. A needle electrode is inserted into the muscle to test the electrical activity, which can be abnormal in muscle disease. Different muscles can be tested to confirm disease distribution.
• A sample of muscle tissue is removed and sent for laboratory analysis. This test may reveal the presence of inflammatory cells around muscle capillaries. This sample can also be checked for the presence of abnormal proteins and enzyme deficiencies.
• Magnetic resonance imaging may be used to scan the body. This creates cross-sectional images of the muscles using magnetic fields and radio waves.
• Esophageal motility is tested using swallowing evaluation and/or esophageal motility studies.
• Lung function may also be tested.