What lab tests are recommended for a patient presenting with symptoms suggestive of myositis, such as muscle weakness, pain, or skin rash?

Laboratory Testing for Myositis

When evaluating a patient with suspected myositis, immediately measure creatine kinase (CK), aldolase, transaminases (AST/ALT), lactate dehydrogenase (LDH), inflammatory markers (ESR, CRP), troponin, and obtain myositis-specific autoantibody testing. 1, 2

Essential Initial Laboratory Panel

Muscle Enzyme Testing

• Creatine kinase (CK) is the primary marker, with levels ≥3× upper limit of normal indicating true inflammatory myositis rather than simple myalgia 1, 3
• Aldolase, LDH, AST, and ALT must be measured because these can be elevated even when CK is normal, particularly in dermatomyositis 2
• All four muscle enzymes should be checked together as part of the complete diagnostic workup 1, 3

Inflammatory Markers

• ESR and CRP are typically elevated in inflammatory myositis and are essential for both diagnosis and monitoring disease activity 1, 2
• Serial monitoring of ESR and CRP helps track treatment response 2

Cardiac Evaluation (Critical)

• Troponin levels must be checked immediately to evaluate for potentially life-threatening myocardial involvement 2, 3
• Obtain ECG emergently, as myocardial involvement dramatically changes prognosis and requires aggressive immunosuppression 3
• Consider echocardiogram or cardiac MRI if troponin is elevated or ECG is abnormal 3

Renal Assessment

• Urinalysis for myoglobinuria screens for rhabdomyolysis, which can be life-threatening 3

Autoantibody Testing

Myositis-Specific Autoantibodies (MSAs)

• Order a comprehensive myositis autoantibody panel including anti-aminoacyl-tRNA synthetases (anti-Jo-1, anti-PL-12, anti-PL-7), anti-SRP, anti-Mi-2, anti-TIF1γ, and anti-NXP2 1, 4
• Autoantibody status is more useful than clinical classification alone in predicting clinical course, prognosis, and treatment response 4
• Each patient typically has only one myositis-specific autoantibody, and these define distinct clinical phenotypes 4

Clinical Significance of Specific Antibodies

• Anti-synthetase antibodies (anti-Jo-1, anti-PL-12) predict arthritis, fever, interstitial lung disease, mechanic's hands, and higher death rates 4
• Anti-SRP antibodies indicate severe, refractory disease with increased myalgias and higher mortality 4
• Anti-Mi-2 antibodies predict cutaneous manifestations (V-sign, shawl-sign rashes) and good response to therapy 4

Important Clinical Caveats

Normal CK Does Not Exclude Myositis

• In dermatomyositis, CK can be normal while aldolase, LDH, AST, and ALT remain elevated 2
• MRI of proximal limbs can reveal inflammatory changes even when muscle enzymes are normal 2
• Muscle biopsy should be considered when diagnosis is uncertain despite normal CK 2

Cardiac Involvement is a Medical Emergency

• Failing to recognize cardiac involvement can be fatal if not detected early 1, 2
• Any elevation in troponin or ECG abnormality requires urgent cardiology consultation 2, 3

Additional Screening

• Malignancy screening is appropriate given the paraneoplastic association, particularly in patients with anti-TIF1γ or anti-NXP2 antibodies 2
• Screen based on age-appropriate guidelines and individual risk factors 2

Monitoring Laboratory Tests

• Serial CK, ESR, and CRP for disease activity tracking 2
• Muscle strength assessments to evaluate treatment response 2
• Repeat troponin and cardiac monitoring if initially abnormal 2