What laboratory tests are recommended for diagnosing rhabdomyolysis and lactic acidosis?

Laboratory Testing for Rhabdomyolysis and Lactic Acidosis

For rhabdomyolysis, immediately check serum creatine kinase (CK), serum creatinine, complete metabolic panel with electrolytes, urinalysis for myoglobinuria, and serum myoglobin; for lactic acidosis, obtain arterial blood gas with pH and bicarbonate, serum lactate, anion gap calculation, and basic metabolic panel. 1, 2, 3

Essential Laboratory Tests for Rhabdomyolysis

Primary Diagnostic Markers

• Serum creatine kinase (CK): The diagnosis is confirmed when CK level is >1000 U/L or at least 5 times the upper limit of normal 3, 4
• Serum myoglobin: Markedly elevated in rhabdomyolysis, though levels can normalize rapidly (within hours to days) making timing critical 3, 4
• Urinalysis for myoglobinuria: Pathognomonic finding showing dark pigmentation, though often absent during testing despite active disease 4, 5

Critical Monitoring Parameters

• Serum creatinine and blood urea nitrogen: Essential for detecting acute kidney injury, which occurs in 33-50% of rhabdomyolysis cases 3, 6
• Complete metabolic panel including:
  • Potassium (hyperkalemia is life-threatening complication) 1, 3
  • Calcium (hypocalcemia common in severe cases) 1
  • Phosphorus (hyperphosphatemia occurs with muscle breakdown) 1, 3
  • Liver enzymes (AST, ALT, LDH released from damaged muscle) 6, 7

Important Clinical Caveat

CK levels >10,000 U/L are traditionally used to diagnose rhabdomyolysis, but studies show healthy individuals can reach these levels after eccentric exercise without renal impairment 7. The key distinction is clinical context—exercise-induced elevations without symptoms rarely require hospitalization, whereas symptomatic rhabdomyolysis with myalgia, weakness, and dark urine demands aggressive treatment regardless of absolute CK value 3, 7, 4.

Essential Laboratory Tests for Lactic Acidosis

Primary Diagnostic Tests

• Arterial blood gas (ABG): Check pH (acidosis defined as pH <7.35), bicarbonate level, and PCO2 to confirm metabolic acidosis 1, 2, 8
• Serum lactate levels:

  • 2 mmol/L = elevated 1, 2

  • 5 mmol/L = abnormal 2, 9

  • 10 mmol/L = serious/life-threatening 2, 9

• Anion gap calculation: (Na - [Cl + CO2]) with >16 indicating lactic acidosis 2, 9

Critical Supporting Tests

• Complete metabolic panel: Includes electrolytes, serum creatinine, and blood urea nitrogen to assess renal function and identify precipitating factors 1, 2, 8
• Hepatic transaminases: Liver disease impairs lactate clearance since the liver is the major site of lactate removal 1, 2
• Creatine kinase: Severe serotonin syndrome and rhabdomyolysis can cause lactic acidosis with elevated CK 1, 2

Alternative When Lactate Unavailable

Standard bicarbonate from arterial blood gases can substitute for lactate measurement, as bicarbonate decreases almost reciprocally with lactate increase 1. This is particularly useful when multiple arterial samples cannot be obtained 1.

Algorithmic Approach to Testing

When Rhabdomyolysis is Suspected

1. Immediate labs (within first hour): CK, serum creatinine, potassium, calcium, phosphorus 3, 6
2. Concurrent testing: Urinalysis for myoglobinuria, serum myoglobin 3, 4
3. Serial monitoring (every 6-12 hours if CK >5000 U/L): Repeat creatinine, electrolytes, CK to track progression 3, 6
4. Calculate McMahon score on admission: Score ≥6 predicts acute kidney injury requiring renal replacement therapy 3

When Lactic Acidosis is Suspected

1. Immediate labs: ABG with pH and bicarbonate, serum lactate, anion gap calculation 2, 8, 9
2. Identify underlying cause: Complete blood count, electrolytes, creatinine, liver function tests 1, 2, 8
3. Repeat lactate within 6 hours: Monitor trend to assess response to treatment 2, 9
4. Additional tests based on clinical context:
   • If sepsis suspected: Blood cultures, procalcitonin 9
   • If mesenteric ischemia suspected: D-dimer, CT angiography 9
   • If medication-induced: Review metformin use with renal function, check for NRTI exposure 2, 9

Common Pitfalls to Avoid

Do not rely on single hematocrit measurements to detect bleeding or assess severity of rhabdomyolysis, as initial values do not accurately reflect blood loss and are confounded by resuscitation fluids 1. Serial measurements are more useful but still have limited sensitivity 1.

Do not ignore lactate elevations even without hypotension—lactate ≥2 mmol/L indicates tissue hypoperfusion and meets criteria for severe sepsis, requiring full evaluation and serial monitoring 2, 9. Persistent hyperlactatemia can indicate occult organ dysfunction even with normal blood pressure 2, 9.

Avoid using lactate-buffered IV solutions in patients with suspected lactic acidosis and liver failure, as they may worsen acidosis due to impaired hepatic lactate clearance 2, 8.