What laboratory tests are used to diagnose muscle inflammation?

Laboratory Tests for Diagnosing Muscle Inflammation

The primary laboratory tests for diagnosing muscle inflammation include muscle enzymes (creatine kinase, lactate dehydrogenase, transaminases, and aldolase), inflammatory markers, and myositis-specific antibodies, supplemented by imaging and electrophysiological studies when necessary. 1, 2

Core Laboratory Tests

Muscle Enzymes

• Creatine kinase (CK) - often markedly elevated in inflammatory myositis; the most sensitive indicator of muscle damage 1
• Lactate dehydrogenase (LDH) - elevated in muscle inflammation 1, 3
• Aspartate aminotransferase (AST/SGOT) and alanine aminotransferase (ALT/SGPT) - can be elevated due to muscle breakdown rather than liver disease 1, 3
• Aldolase - may be elevated even when CK is normal in some cases of myositis 1, 3

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR) - typically elevated in inflammatory muscle conditions 1
• C-reactive protein (CRP) - elevated in active muscle inflammation 1

Autoantibody Testing

• Myositis-specific antibodies - help classify specific forms of inflammatory myopathies 1, 2
• Myositis-associated antibodies - may indicate overlap syndromes 1, 2
• Anti-nuclear antibodies (ANA) - often positive in inflammatory myopathies 1, 4

Additional Laboratory Evaluation

Complete Blood Count

• Full blood count and blood film - to evaluate for associated cytopenias or evidence of systemic inflammation 1

Metabolic and Organ Function Tests

• Renal function tests - to assess kidney involvement and establish baseline before treatment 1
• Liver function tests - to differentiate between muscle-derived and liver-derived enzyme elevations 1
• Thyroid function tests - to rule out thyroid disorders that can mimic myopathy 1
• Vitamin D levels - deficiency can cause myalgia that may mimic inflammatory conditions 1

Specialized Testing When Initial Results Are Inconclusive

Cardiac Assessment

• Troponin - to evaluate for cardiac muscle involvement, which can be life-threatening 1
• ECG and echocardiogram - recommended for all patients with suspected inflammatory myopathy 1

Pulmonary Assessment

• Pulmonary function tests with CO diffusion - to evaluate for interstitial lung disease, which can accompany myositis 1

Diagnostic Algorithm

1. First-line laboratory tests:

   • Muscle enzymes (CK, LDH, AST, ALT, aldolase) 1, 2
   • Inflammatory markers (ESR, CRP) 1
   • Complete blood count 1
   • Basic metabolic panel and liver function tests 1

2. If muscle inflammation is suspected based on initial tests:

   • Myositis-specific and myositis-associated antibodies 1, 2
   • Consider infection screen to rule out infectious causes 1
   • Evaluate for alternative systemic causes (thyroid, electrolyte disturbances) 1

3. Additional testing based on clinical presentation:

   • MRI of muscles - shows inflammation on T2-weighted/STIR sequences 1, 2
   • Electromyography (EMG) - particularly if neuropathy or neuromuscular junction disorder is suspected 1
   • Muscle biopsy - especially in atypical presentations or absence of rash 1

Important Considerations

• CK levels may be markedly elevated (often >10 times normal) in active inflammatory myositis, but can sometimes be normal despite active disease 2, 3
• Differential diagnosis includes non-inflammatory causes of elevated muscle enzymes such as statin-induced myopathy, metabolic myopathies, and muscular dystrophies 1
• Muscle biopsy remains the gold standard for diagnosis in atypical cases 1
• Nailfold capillaroscopy can aid in diagnosis of dermatomyositis 1
• Early diagnosis is critical to prevent irreversible muscle damage and improve outcomes 1, 5