Which laboratory finding best differentiates polymyositis from dermatomyositis?

• A. CK
• B. Aldolase
• C. ESR
• D. Anti-SRP

Answer: (D)

The important distinction between polymyositis and dermatomyositis lies in their autoantibody profiles, which helps separate the two when clinical features are similar. Dermatomyositis is associated with antibodies that are relatively specific for it, especially anti-Mi-2. The presence of anti-Mi-2 strongly supports dermatomyositis because these antibodies are not typically found in polymyositis. In contrast, polymyositis more often has other antibodies such as anti-Jo-1 (anti-synthetase), and anti-Mi-2 is usually absent. Anti-SRP is linked to a different variant called necrotizing autoimmune myopathy and is not characteristic of either DM or PM, so it’s not useful for differentiating between them.

Beyond antibodies, biopsy patterns can help: dermatomyositis typically shows perifascicular atrophy with perivascular and inflammatory changes, while polymyositis shows endomysial inflammation with muscle fiber necrosis but without the perifascicular pattern. Serum CK and ESR can be elevated in both conditions and aren’t reliable differentiators.