What labs are used to diagnose and manage rhabdomyolysis (muscle breakdown disease)?

Diagnostic Laboratory Tests for Rhabdomyolysis

Serum creatine kinase (CK) is the primary diagnostic laboratory test for rhabdomyolysis, with a diagnostic threshold of >1000 U/L or at least 5 times the upper limit of normal. 1

Essential Laboratory Tests

First-Line Tests

• Serum creatine kinase (CK)

  • Gold standard for diagnosis
  • Peaks 24-36 hours after muscle injury
  • May reach levels >100,000 U/L in severe cases 2

• Plasma/serum myoglobin

  • Rises earlier than CK (1-3 hours after injury)
  • Shorter half-life (2-3 hours) than CK
  • May return to normal while CK remains elevated 1

• Urinalysis

  • Check for myoglobinuria (tea/cola-colored urine)
  • Positive for blood on dipstick but negative for RBCs on microscopy 1

• Renal function tests

  • Blood urea nitrogen (BUN)
  • Serum creatinine
  • eGFR to assess kidney function 1

• Serum electrolytes

  • Potassium (critical for monitoring hyperkalemia)
  • Calcium (hypocalcemia may occur)
  • Phosphorus (hyperphosphatemia may occur)
  • Magnesium 1

Additional Important Tests

• Liver function tests

  • AST/ALT (often elevated due to muscle breakdown)
  • Helps differentiate from primary liver disease 1

• Coagulation studies

  • PT/PTT/INR if disseminated intravascular coagulation is suspected 1

• Arterial blood gas

  • To assess for metabolic acidosis 1

Monitoring Parameters

• Serial CK measurements

  • Every 6-12 hours in acute phase
  • Single measurements can miss peak values 1

• Electrolyte monitoring

  • Daily or more frequently if abnormal
  • Potassium levels require close monitoring due to risk of hyperkalemia 1

• Urine output

  • Target >300 mL/hour during aggressive hydration
  • Hourly measurement in acute phase 1

• Urine pH

  • Target >6.5 if alkalinization strategy is used 1

Specialized Testing for Recurrent or Unexplained Cases

When rhabdomyolysis occurs with minimal exertion, has recurrent episodes, begins in childhood/adolescence, or has a family history, consider:

• Metabolic screening

  • Fasting blood glucose
  • Lactate and pyruvate levels
  • Acylcarnitine profile
  • Urine organic acids
  • Plasma amino acids 1

• Muscle biopsy

  • For recurrent episodes without identified cause
  • When inherited myopathy is suspected 1

• Genetic testing

  • For glycogen storage diseases
  • Fatty acid oxidation disorders
  • Mitochondrial disorders
  • Muscular dystrophies
  • Ion channelopathies 1

Common Pitfalls to Avoid

1. Relying on single CK measurements - Serial monitoring is essential as peaks may be missed 1

2. Missing myoglobinuria - Urine may appear normal if tested late (myoglobin has short half-life) 1

3. Confusing with myositis - Both have elevated CK, but inflammatory markers are typically higher in myositis 1

4. Overlooking genetic causes in adults with first-time rhabdomyolysis 1

5. Failing to test for compartment syndrome - Consider pressure measurement when pressure >30 mmHg or differential pressure <30 mmHg 1

By following this comprehensive laboratory approach, clinicians can effectively diagnose rhabdomyolysis, monitor for complications, and identify underlying causes to guide appropriate management.