What lab workup is recommended for a patient presenting with muscle weakness?

From the Guidelines

For a patient presenting with muscle weakness, a comprehensive laboratory workup should include creatine kinase (CK), thyroid function tests (TSH), and electrolytes, as these tests can help identify potential causes of weakness, including muscular dystrophy and thyroid disorders, as recommended by 1 and 1. When evaluating a patient with muscle weakness, it is essential to consider various potential causes, including electrolyte disturbances, endocrine disorders, inflammatory myopathies, metabolic myopathies, and neuromuscular junction disorders.

• The initial laboratory workup should include:
  • Creatine kinase (CK) to evaluate for muscular dystrophy, as recommended by 1
  • Thyroid function tests (TSH) to evaluate for thyroid disorders, as recommended by 1 and 1
  • Electrolytes, including calcium, magnesium, and phosphorus, to evaluate for electrolyte disturbances
• Additional testing may include:
  • Vitamin D levels, as recommended by 1
  • Aldolase and lactate dehydrogenase (LDH) to evaluate for metabolic myopathies
  • Urinalysis to evaluate for renal function and potential electrolyte disturbances
• If autoimmune conditions are suspected, further testing may include:
  • Antinuclear antibody (ANA) and rheumatoid factor (RF) to evaluate for autoimmune disorders
  • Specific myositis antibodies to evaluate for inflammatory myopathies
• Depending on the clinical presentation, consider testing for:
  • Myasthenia gravis with acetylcholine receptor antibodies
  • Diabetes with HbA1c
• In cases of persistent unexplained weakness despite initial testing, specialized tests like electromyography (EMG), nerve conduction studies, or muscle biopsy may be necessary, as recommended by 1 and 1. The pattern of muscle weakness, onset, and associated symptoms should guide the specific tests ordered, as recommended by 1.

From the Research

Lab Workup for Muscle Weakness

The lab workup for a patient presenting with muscle weakness involves a stepwise approach to narrow down the differential diagnosis. The following tests are recommended:

• Electrolyte measurements to evaluate for electrolyte imbalances 2, 3
• Thyroid-stimulating hormone (TSH) assay to evaluate for endocrine causes 2, 3
• Creatine kinase (CK) level to evaluate for muscle damage 2, 3, 4, 5
• Electromyography (EMG) to confirm the presence of a myopathy or to evaluate for a neuropathy or a disease of the neuromuscular junction 2, 3, 6, 5
• Magnetic resonance imaging (MRI) if there is concern for acute neurologic conditions, such as stroke or cauda equina syndrome, and may also guide muscle biopsy 2
• Muscle biopsy to evaluate for myopathic changes and to guide diagnosis 2, 3, 4, 6

Additional Tests

Additional tests that may be helpful in evaluating muscle weakness include:

• Erythrocyte sedimentation rate (ESR) and antinuclear antibody (ANA) assays to evaluate for rheumatologic, inflammatory, genetic, and metabolic causes 3
• Genetic testing to evaluate for genetic causes of muscle weakness 4
• Blood chemistries to evaluate for electrolyte and endocrine causes 3

Approach to Diagnosis

The approach to diagnosis of muscle weakness involves a combination of history, physical examination, and laboratory testing. The patient's history and physical examination should be used to determine the pattern and severity of weakness, associated symptoms, and potential causes of weakness. Laboratory testing should be guided by the patient's history and physical examination, and should include a stepwise approach to narrow down the differential diagnosis 2, 3, 4, 5.