What labs should be drawn to assess for rhabdomyolysis (muscle breakdown)?

Laboratory Tests for Assessing Rhabdomyolysis

To diagnose rhabdomyolysis, the most essential laboratory tests are serum creatine phosphokinase (CPK), serum myoglobin, and urinalysis for myoglobinuria. 1

Core Laboratory Tests

Primary Diagnostic Tests

• Serum Creatine Phosphokinase (CPK/CK)

  • Diagnostic criterion: >1000 U/L or at least 5 times the upper limit of normal 2
  • Most reliable marker for diagnosis and monitoring
  • Serial measurements recommended to track progression and recovery

• Serum Myoglobin

  • Rises earlier than CPK but clears more rapidly
  • Important for early detection
  • Less reliable for monitoring due to shorter half-life

• Urinalysis

  • Check for myoglobinuria (appears as blood on dipstick with few or no RBCs on microscopy)
  • Urine pH (maintain at 6.5 to prevent tubular cast formation) 1

Essential Electrolyte and Metabolic Panel

• Complete Metabolic Panel
  • Serum creatinine and BUN (to assess kidney function)
  • Potassium (often elevated due to muscle breakdown)
  • Calcium (often decreased initially, may rebound later)
  • Phosphorus (typically elevated)
  • Sodium (to monitor fluid status)

Additional Tests Based on Clinical Presentation

• Acid-Base Status

  • Arterial or venous blood gas to assess for metabolic acidosis

• Coagulation Studies

  • If disseminated intravascular coagulation is suspected

• Inflammatory Markers

  • ESR and CRP if autoimmune myositis is suspected 1

Special Considerations

Risk Assessment

• Calculate McMahon score on admission (score ≥6 predicts need for renal replacement therapy) 2

Monitoring

• Serial CPK measurements until levels decrease below 1,000 U/L 3
• Hourly urine output monitoring via bladder catheterization 1

Important Caveats

1. False Negatives: Normal CPK does not rule out rhabdomyolysis in all cases. Some patients may present with normal CPK levels despite having rhabdomyolysis-induced AKI 4

2. Timing of Tests: CPK typically peaks 24-36 hours after muscle injury and may be missed if testing is delayed or too early

3. Confounding Factors: Certain conditions like diabetic emergencies can present with rhabdomyolysis and may require additional testing 5

4. Extreme Values: In severe cases, CPK can reach extremely high levels (>200,000 or even near 1 million U/L) 6

By systematically ordering these laboratory tests, clinicians can promptly diagnose rhabdomyolysis, assess its severity, monitor for complications (especially acute kidney injury), and guide appropriate fluid resuscitation therapy.