What lab tests are recommended for a patient suspected of having myositis?

Laboratory Testing for Suspected Myositis

For any patient with suspected myositis, immediately obtain creatine kinase (CK), aldolase, transaminases (AST, ALT), lactate dehydrogenase (LDH), troponin, and inflammatory markers (ESR, CRP) as your essential first-line panel. 1, 2

Core Laboratory Panel

Muscle Enzyme Testing

• Creatine kinase (CK) is the primary marker for muscle inflammation and distinguishes myositis from other causes of weakness 1, 3
• Aldolase can be elevated even when CK is normal and may indicate damage to early regenerating muscle cells, making it particularly valuable in dermatomyositis 1, 2, 4
• Transaminases (AST, ALT) are frequently elevated in myositis because muscle injury—not liver disease—releases these enzymes 1, 3
• Lactate dehydrogenase (LDH) serves as an additional marker of muscle inflammation 1, 2

Cardiac Assessment

• Troponin levels are mandatory to evaluate for potentially life-threatening myocardial involvement, which requires urgent treatment and permanent discontinuation of any causative agents 1, 2
• Obtain an echocardiogram if troponin is elevated or if there is any clinical suspicion of cardiac involvement 1, 2

Inflammatory Markers

• ESR and CRP are essential for both initial diagnosis and serial monitoring of disease activity 1, 2

Autoantibody Testing

Myositis-Specific Antibodies

• Autoimmune myositis panel should include testing for myositis-specific autoantibodies, as approximately 60% of patients with inflammatory myopathies have identifiable antibodies 1, 5
• Consider paraneoplastic autoantibody testing for myositis, particularly in dermatomyositis where malignancy association is significant 1, 2

Neurologic Overlap Testing

• If myasthenia gravis is suspected (concomitant presentation carries high mortality), test for anti-acetylcholine receptor and anti-striated muscle antibodies 1

Monitoring Laboratory Tests

Once myositis is diagnosed, serial monitoring should include:

• CK levels to track muscle inflammation 1, 2
• ESR and CRP for disease activity assessment 1, 2
• Repeat testing every 2-4 weeks initially, then every 4-6 weeks after treatment is instituted 1, 6

Critical Diagnostic Pitfalls

Do not pursue extensive hepatic evaluation (including liver biopsy) in patients with elevated transaminases and any clinical evidence of muscle disease until CK has been checked—this prevents diagnostic delays and misdiagnosis of primary liver disease 3

Normal CK does not exclude myositis, particularly in dermatomyositis where aldolase, LDH, AST, and ALT may be elevated despite normal CK 2, 4

Elevated CK with muscle weakness is more concerning than elevated CK with pain alone—muscle weakness is the hallmark of true myositis, while pain without weakness may indicate polymyalgia rheumatica-like syndrome where CK should be normal 1