Serum Creatine Kinase (CK) is the Primary Blood Test for Diagnosing Rhabdomyolysis
The diagnosis of rhabdomyolysis is confirmed when serum creatine kinase (CK) levels exceed 1000 U/L or at least 5 times the upper limit of normal. 1 This elevation in CK is the most sensitive and specific laboratory marker for muscle breakdown and is considered the gold standard for diagnosis.
Diagnostic Approach to Rhabdomyolysis
Primary Diagnostic Test
- Serum creatine kinase (CK): The definitive diagnostic test for rhabdomyolysis
- Diagnostic threshold: >1000 U/L or ≥5× upper limit of normal
- Peak levels typically occur 24-36 hours after muscle injury
- Serial measurements help monitor disease progression and response to treatment
Supporting Laboratory Tests
Serum myoglobin: Released from damaged muscle cells
- Rises earlier than CK (1-3 hours after injury)
- Clears more rapidly (half-life of 2-3 hours vs. 1.5 days for CK)
- Less sensitive than CK for diagnosis due to rapid clearance
Urinalysis: To detect myoglobinuria
- Positive dipstick for blood without RBCs on microscopy
- Red-brown or tea-colored urine
- Note: Myoglobinuria may be absent if myoglobin levels are low or kidney function is impaired
Complete metabolic panel:
- Electrolyte abnormalities (hyperkalemia, hypocalcemia, hyperphosphatemia)
- Elevated BUN/creatinine ratio (indicates kidney injury)
- Elevated liver enzymes (AST, ALT) from muscle breakdown
Clinical Presentation and Risk Assessment
Clinical Features
- Muscle pain, weakness, or swelling
- Dark urine (tea or cola-colored)
- Systemic symptoms: fever, malaise, nausea, vomiting
- Signs of volume depletion or kidney injury
Risk Stratification
- McMahon score ≥6 on admission predicts need for renal replacement therapy 1
- Monitor for complications:
- Acute kidney injury
- Compartment syndrome
- Electrolyte disturbances
- Disseminated intravascular coagulation
Management Considerations
Immediate Interventions
- Aggressive fluid resuscitation with isotonic crystalloids
- Target urine output of 200-300 mL/hour 2
- Daily target of at least 6L for severe cases
Monitoring
- Serial CK measurements to track muscle injury
- Electrolytes every 4-6 hours initially 2
- Renal function tests
- Acid-base status
- Compartment pressures if suspected
Common Pitfalls to Avoid
Delayed diagnosis: Always consider rhabdomyolysis when patients present with muscle pain and dark urine, especially after exercise, trauma, or medication changes 3
Misdiagnosis: Symptoms may be misattributed to other conditions, including psychiatric disorders 3
Inadequate fluid resuscitation: Underestimating fluid requirements can lead to preventable kidney injury
Overlooking compartment syndrome: Regular neurovascular checks are essential 2
Failure to identify and address the underlying cause: Treatment must include addressing the primary trigger of muscle breakdown
Remember that while CK is the definitive diagnostic test for rhabdomyolysis, a comprehensive approach including supporting laboratory tests and clinical assessment is essential for effective management and prevention of complications.