What is the initial workup for a patient suspected of having rhabdomyolysis (muscle breakdown)?

Rhabdomyolysis Initial Workup

The initial workup for suspected rhabdomyolysis requires immediate measurement of creatine kinase (CK), comprehensive electrolyte panel including potassium, renal function tests (creatinine and BUN), urinalysis for myoglobinuria, and arterial blood gas to assess for metabolic acidosis. 1, 2, 3

Core Laboratory Tests

Essential Immediate Tests

• Creatine kinase (CK): A level ≥5-10 times the upper limit of normal (typically >1,000 U/L) is diagnostic for rhabdomyolysis 2, 3, 4
• Complete electrolyte panel including:
  • Potassium (hyperkalemia is life-threatening and can cause cardiac arrhythmias) 1
  • Calcium (hypocalcemia is common) 5
  • Phosphorus (hyperphosphatemia occurs with muscle breakdown) 5
  • Magnesium 1
• Renal function tests: Creatinine and BUN to assess for acute kidney injury, the most significant complication 5, 2, 3
• Urinalysis: Check for myoglobinuria (red-to-brown urine without red blood cells on microscopy) 2, 6
• Arterial blood gas: Essential to assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis 1

Additional Monitoring Tests

• Plasma myoglobin: Should be measured serially along with CK and potassium 1
• Liver function tests: Necessary to assess hepatic involvement and distinguish injury patterns 1
• Coagulation studies: Evaluate for disseminated intravascular coagulation, a potential complication of severe rhabdomyolysis 1
• Electrocardiogram: Mandatory to identify cardiac arrhythmias from hyperkalemia 1
• Cardiac troponin: Should be performed in severe cases to rule out cardiac involvement 1

Clinical Assessment

Key History and Physical Examination Findings

• Classic triad symptoms (though often absent): Muscle pain, weakness, and dark/tea-colored urine 2, 6, 4
• Early signs of compartment syndrome: Pain, tension, paresthesia, and paresis in affected limbs 1
• Late signs of compartment syndrome: Pulselessness and pallor (indicate irreversible damage) 1
• Risk factor identification:
  • Trauma or crush injuries 1, 6
  • Exertional causes (novel overexertion, unaccustomed exercise) 1
  • Medications (particularly statins) 1
  • Prolonged immobilization 7
  • Intoxication or altered mentation 7

Etiology-Specific Testing

When Clinically Indicated

• Medication review: Identify causative agents, particularly statins and drug interactions 1
• Viral studies: Consider if viral myositis is suspected 1
• Autoimmune markers (ANA, ASMA, ANCA): Evaluate if autoimmune myositis is suspected 1
• Toxicology screen: In cases of suspected drug or toxin exposure 6, 7

Monitoring Frequency

Serial measurements every 6-12 hours of CK, electrolytes (especially potassium), and renal function are essential during the acute phase 5. The Anaesthesia guideline society emphasizes that repeated bioassessment including plasma myoglobin, CK, and potassium is critical 1.

Critical Pitfalls to Avoid

• Do not delay fluid resuscitation while waiting for laboratory results—early aggressive IV hydration is critical to prevent acute kidney injury 1, 2, 3
• Do not miss hyperkalemia: Obtain ECG immediately as hyperkalemia can cause life-threatening arrhythmias 1, 7
• Do not overlook compartment syndrome: Monitor compartment pressures if clinical suspicion exists; fasciotomy is indicated when pressure exceeds 30 mmHg or differential pressure (diastolic BP - compartment pressure) is <30 mmHg 1
• Do not routinely measure CK and transaminases in asymptomatic statin users—this is not recommended for screening 8