Diagnosis and Monitoring of Polymyositis

Clinical Presentation and Diagnosis

The American College of Rheumatology recommends that elevation of muscle enzymes, including creatine kinase (CK), is one of the central diagnostic criteria for polymyositis, along with proximal muscle weakness, electromyographic changes, and inflammatory infiltrate on muscle biopsy 1.
The European League Against Rheumatism suggests that CK levels can be elevated up to 15 times the upper limit of normal in active polymyositis, and this can be used to assess therapeutic response 2, 3.

The American College of Rheumatology notes that immune-mediated necrotizing myopathy (IMNM) typically presents with much higher CK elevations, often more than 10 times the upper limit of normal, with acute or subacute onset of severe proximal weakness 1, 4.
The European League Against Rheumatism states that polymyalgia rheumatica syndrome presents with severe myalgia but normal CK levels, distinguishing it from true polymyositis 5.

The American College of Rheumatology recommends that patients with markedly elevated CK (>3x normal) and weakness should immediately start high-dose corticosteroids and be evaluated for potentially fatal complications such as myocarditis 5.
The European League Against Rheumatism suggests that a decrease of >40% in CK levels corresponds to a 7.5-point improvement in the therapeutic response score according to the 2016 ACR/EULAR criteria 2, 3.

The American College of Rheumatology notes that when clinical suspicion is high but CK is normal or only slightly elevated, measuring additional muscle enzymes (aldolase, AST, ALT, LDH) may be helpful, as one may be elevated when CK is normal 5, 4.
The European League Against Rheumatism states that muscle biopsy remains a crucial diagnostic tool when the diagnosis is uncertain, even with elevated CK 5.