What blood tests are used to diagnose myopathy?

Blood Tests for Diagnosing Myopathy

The primary blood tests for diagnosing myopathy include muscle enzymes such as creatine phosphokinase (CPK), lactate dehydrogenase (LDH), aspartate aminotransferase (AST/SGOT), alanine aminotransferase (ALT/SGPT), and aldolase, which should be ordered in all patients with suspected myopathy. 1

Core Laboratory Investigations

Muscle Enzymes

• Creatine phosphokinase (CPK) - Primary marker for muscle damage
• Lactate dehydrogenase (LDH) - Elevated in various myopathies
• Aspartate aminotransferase (AST/SGOT) - Can be elevated with muscle breakdown
• Alanine aminotransferase (ALT/SGPT) - May increase with muscle damage
• Aldolase (if available) - Often elevated in inflammatory myopathies

Additional Essential Blood Tests

• Complete blood count and blood film - To evaluate for systemic inflammation or other conditions
• Erythrocyte sedimentation rate (ESR) or plasma viscosity and C-reactive protein (CRP) - Inflammatory markers
• Myositis-specific and myositis-associated antibodies - For diagnosing inflammatory myopathies
• Renal function and liver function tests - To assess organ involvement and rule out other causes
• Thyroid function tests - To rule out endocrine myopathies 1

Specialized Testing Based on Clinical Suspicion

For Inflammatory Myopathies

• Anti-Jo-1 antibodies - Associated with antisynthetase syndrome
• Anti-SRP antibodies - Associated with necrotizing myopathy
• Anti-Mi2 antibodies - Associated with dermatomyositis
• Anti-p155/140 kDa protein antibodies - May be associated with juvenile dermatomyositis 1

For Metabolic Myopathies

• Vitamin D levels - To rule out vitamin D deficiency myopathy
• Electrolyte panel - To identify electrolyte disturbances causing myopathy
• Lactate and pyruvate levels - For suspected mitochondrial myopathies
• Genetic testing - For suspected muscular dystrophies or mitochondrial disorders 1, 2

Diagnostic Algorithm

1. Initial screening:

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

2. If inflammatory myopathy suspected (based on elevated enzymes, inflammatory markers):

   • Myositis-specific and myositis-associated antibodies
   • Consider MRI of affected muscles
   • Plan for EMG and muscle biopsy

3. If metabolic myopathy suspected:

   • Lactate and pyruvate levels
   • Consider cardiopulmonary exercise testing
   • Specific genetic testing based on clinical presentation 2

Important Considerations

• Blood tests alone are insufficient for definitive diagnosis and should be combined with clinical assessment, EMG, and often muscle biopsy 1

• Muscle biopsy remains the gold standard for confirming inflammatory myopathies and distinguishing between inflammatory and non-inflammatory myopathies 1, 3

• False negatives can occur with blood testing, particularly in chronic or treated myopathies where enzyme levels may normalize despite ongoing disease 4

• Electromyography (EMG) should be performed to confirm myopathic processes and help target muscles for biopsy 1

• Consider whole mitochondrial genome sequencing when mitochondrial myopathy is suspected, particularly when cardiopulmonary exercise testing shows decreased oxygen consumption and elevated VE/VO₂ ratio 2

Remember that while blood tests are an essential component of myopathy diagnosis, they should be interpreted in the context of clinical presentation, imaging findings, EMG results, and often muscle biopsy for definitive diagnosis.