First of all, it is important to realize that there are no specific diagnostic tests for dermatomyositis at this point in time. However, there is certainly a constellation of a number of blood tests that helps in that case to make the diagnosis of dermatomyositis in combination with the clinical signs and symptoms.
Secondly, as in other connective tissue disease there often is an elevation of the ESR (sedimentation rate).
Other connective tissue diseases may be present
Thirdly, if other connective tissue diseases are also present, surely the ANA titer is often positive. One other immunological test that has been especially useful in recent years is the Jo-1 antibody test. These are autoantibodies against thymus antigens, which an ELISA type screening test can detect (Ref.2). In effect, a high percentage of patients with a positive Jo-1 antibody test have dermatomyositis.
Furthermore, the literature has shown that these types of autoantibodies seem to be present in particular in patients who develop pulmonary complications (interstitial pneumonitis), polyarthritis symptoms or subcutaneous calcifications (Ref. 3 and 4). The muscle enzymes (transaminases, creatinine kinase (CK), aldolase) are notably markedly elevated and are useful to follow to monitor the success of treatment (Ref. 1, p.435).
Muscle biopsy, if in doubt
Finally, by all means, if the physician finds that there are still doubts about the diagnosis, a muscle biopsy can be obtained, which is always reliable and indeed is the most definitive test. Electromyography studies, also called EMG tests, can aid in the diagnosis as there are for example fairly characteristic changes visible on this test.
Another test, an MRI scan can occasionally help to identify an area with edematous and inflammatory changes where a muscle biopsy for instance can then lead to the definitive diagnosis of dermatomyositis. Even though only 15% of male patients will have an occult cancer in general, the physician needs to do cancer screening tests in all patients with dermatomyositis, just to play it safe.
Given these points it is clear that dermatomyositis is a diagnosis that often requires skill, intuition and many different tests to confirm it.
1.The Merck Manual, 7th edition, by M. H. Beers et al., Whitehouse Station, N.J., 1999. Chapter 50.
2. WA Schmidt et al. Clin Rheumatol 2000;19(5):371-377.
3. A Sauty et al. Eur Respir J 1997 Dec;10(12):2907-2912.
4. R Queiro-Silva et al. J Rheumatol 2001 Jun;28(6):1401-1404.
5. J Wada et al. Clin Exp Immunol 2001 May;124(2):282-289.
6. Ferri: Ferri’s Clinical Advisor: Instant Diagnosis and Treatment, 2004 ed., Copyright © 2004 Mosby, Inc.
7. Rakel: Conn’s Current Therapy 2004, 56th ed., Copyright © 2004 Elsevier