What labs are recommended for diagnosing and managing rhabdomyolysis (rhabdo)?

Laboratory Tests for Diagnosing and Managing Rhabdomyolysis

The essential laboratory tests for diagnosing and monitoring rhabdomyolysis include serum creatine kinase (CK) levels (at least 10 times the upper limit of normal), renal function tests, electrolytes, and urinalysis for myoglobinuria. 1

First-Line Diagnostic Tests

• Serum Creatine Kinase (CK): Primary diagnostic marker

  • Diagnostic threshold: ≥10 times upper limit of normal (can reach >100,000 IU/L)
  • Peaks 24-72 hours after muscle injury 1, 2

• Renal Function Tests:

  • Blood urea nitrogen (BUN)
  • Serum creatinine
  • eGFR (estimated glomerular filtration rate) 1

• Electrolytes:

  • Potassium (monitor for hyperkalemia)
  • Calcium (monitor for hypocalcemia)
  • Phosphorus (monitor for hyperphosphatemia) 1

• Urinalysis:

  • Myoglobinuria (note: may be negative in up to 81% of cases using qualitative assays) 3
  • Urine dipstick (positive for blood without RBCs on microscopy suggests myoglobinuria) 1

Additional Recommended Tests

• Liver Function Tests:

  • AST/ALT (often elevated due to muscle breakdown)
  • Alkaline phosphatase 4, 1

• Complete Blood Count 4

• Inflammatory Markers:

  • ESR (erythrocyte sedimentation rate)
  • CRP (C-reactive protein) 4

• Coagulation Studies:

  • PT/INR
  • aPTT (to assess for DIC risk) 1

• Acid-Base Status:

  • Arterial blood gases (if severe)
  • Serum lactate 1

Specialized Tests for Differential Diagnosis

• Serum Myoglobin: Confirms muscle breakdown but has shorter half-life (2-3 hours) than CK 1

• Creatine Kinase Isoenzymes: To differentiate muscle (CK-MM) from cardiac (CK-MB) origin 1

• ECG: To assess cardiac complications and effects of electrolyte abnormalities 1

Tests for Identifying Underlying Causes

For recurrent or unexplained cases, consider:

• Muscle Biopsy: Especially in atypical presentations or suspected metabolic myopathies 4, 1

• Genetic Testing: For recurrent cases or suspected inherited disorders 1

• Additional Tests Based on Clinical Suspicion:

  • Toxicology screen (alcohol, illicit drugs)
  • Medication levels (statins, etc.)
  • Viral studies (influenza, HIV, etc.)
  • Thyroid function tests 4, 1

Monitoring Parameters

• Serial CK Measurements: To track resolution (typically decreases by ~40% daily when cause is removed) 1

• Daily Renal Function: BUN, creatinine 1

• Electrolytes: Every 4-6 hours initially in severe cases 1

• Urine Output: Goal >300 mL/hour during acute phase 1

Important Clinical Considerations

• The classic triad of myalgia, muscle weakness, and pigmenturia is present in less than 10% of cases 1, 2

• Risk of acute kidney injury increases significantly when CK >16,000 IU/L 2

• Urine myoglobin testing has poor sensitivity (positive in only 19% of cases) 3

• Monitor for compartment syndrome in traumatic causes (measure compartment pressures if suspected) 1

• Consider nephrology consultation for all cases with acute kidney injury 1

By systematically ordering and monitoring these laboratory tests, clinicians can effectively diagnose rhabdomyolysis, assess its severity, identify complications, and guide appropriate management decisions.