What are the diagnostic tests and interventions for rhabdomyolysis?

Diagnostic Testing for Rhabdomyolysis

The essential diagnostic test for rhabdomyolysis is serum creatine kinase (CK) elevation, typically at least 10 times the upper limit of normal (approximately 1000 IU/L), along with urinalysis showing positive blood on dipstick but negative for RBCs on microscopy. 1

Primary Diagnostic Tests

• Serum creatine kinase (CK)

  • Diagnostic threshold: >10x upper limit of normal (approximately 1000 IU/L)
  • Peak levels may occur 24-72 hours after muscle injury
  • Serial monitoring recommended every 6-12 hours in acute phase

• Urinalysis

  • Check for myoglobinuria (tea/cola-colored urine)
  • Dipstick positive for blood but negative for RBCs on microscopy
  • Urine pH monitoring if alkalinization strategy is used (target >6.5)

• Plasma myoglobin

  • Confirms muscle breakdown
  • Rises earlier than CK but has shorter half-life (2-3 hours)
  • May return to normal while CK remains elevated

Additional Essential Laboratory Tests

• Renal function tests

  • BUN and creatinine to assess kidney function
  • Monitor for acute kidney injury

• Electrolytes

  • Potassium (critical for monitoring hyperkalemia)
  • Calcium (hypocalcemia may occur early)
  • Phosphorus (hyperphosphatemia)
  • Magnesium

• Liver function tests

  • AST/ALT may be elevated due to muscle breakdown
  • Helps differentiate from primary liver disease

• Acid-base status

  • Arterial blood gas to assess for metabolic acidosis
  • pH monitoring if bicarbonate therapy is used

Monitoring Parameters

• CK levels: Monitor every 6-12 hours in acute phase until declining
• Electrolytes: Monitor daily or more frequently if abnormal
• Renal function: Monitor BUN and creatinine every 6-12 hours initially
• Urine output: Measure hourly in acute phase (target >300 mL/hour)

Special Considerations for Recurrent or Unexplained Rhabdomyolysis

For patients with recurrent episodes, episodes triggered by specific factors, rhabdomyolysis with minimal exertion, onset during childhood/adolescence, or family history of similar episodes:

• Metabolic screening:

  • Fasting blood glucose
  • Lactate and pyruvate levels
  • Acylcarnitine profile
  • Urine organic acids
  • Plasma amino acids

• Specific testing for inherited disorders:

  • Forearm ischemic exercise test
  • Muscle glycogen content analysis
  • Specific enzyme assays
  • Genetic testing panels

• Muscle biopsy: Consider when there are recurrent episodes without identified cause or strong suspicion of inherited myopathy

Differential Diagnosis Considerations

• Compartment syndrome: Measure compartment pressure if >30 mmHg or differential pressure (diastolic BP - compartment pressure) <30 mmHg 2, 1

  • Clinical signs: pain, tension, paresthesia, paresis
  • Note: clinical signs have low sensitivity but high negative predictive value

• Myositis vs. Rhabdomyolysis: Both have elevated CK, but inflammatory markers are typically higher in myositis; autoantibody testing may be needed 1

Common Pitfalls to Avoid

• Relying on single CK measurements (can miss peak values)
• Delaying fluid resuscitation while waiting for laboratory results
• Using potassium-containing fluids for resuscitation (may worsen hyperkalemia)
• Missing genetic causes in adults with first-time rhabdomyolysis
• Attributing recurrent episodes solely to exercise or dehydration
• Failing to perform metabolic studies during acute episodes

Remember that prompt recognition and aggressive fluid management are crucial to preventing complications, particularly acute kidney injury. Serial monitoring of laboratory values is essential throughout the course of treatment.