Diagnosis of Rhabdomyolysis
Diagnose rhabdomyolysis when creatine kinase (CK) is elevated at least 10 times the upper limit of normal, followed by a rapid decrease toward normal values. 1, 2
Core Diagnostic Criteria
The primary diagnostic marker is serum CK elevation ≥10 times the upper limit of normal, which is the most sensitive laboratory finding for muscle injury. 1, 2, 3, 4 While the classic triad of myalgia, weakness, and pigmenturia (tea-colored urine) is often cited, this combination appears in less than 10% of patients, making it unreliable for diagnosis. 2
Essential Laboratory Tests
Obtain the following tests immediately when rhabdomyolysis is suspected:
- Serum creatine kinase (CK): The cornerstone diagnostic test; levels may reach 100,000 IU/L or higher in severe cases 1, 2
- Serum myoglobin: Released earlier than CK but has a shorter half-life, making it useful for early detection 1, 5
- Complete electrolyte panel: Assess for hyperkalemia (life-threatening), hypocalcemia, and hyperphosphatemia 1, 5
- Renal function tests: BUN and creatinine to detect acute kidney injury, which occurs particularly when CK >15,000-16,000 IU/L 1, 2
- Urinalysis: Look for brown/cloudy urine that is positive for blood without red blood cells, indicating myoglobinuria 1, 3
Additional Diagnostic Tests Based on Severity
For moderate to severe cases, expand the workup:
- Arterial blood gas: Assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis 1, 5
- Coagulation studies: Evaluate for disseminated intravascular coagulation in severe cases 1, 5
- ECG and cardiac troponin: Rule out cardiac involvement and detect arrhythmias from electrolyte abnormalities 1, 5
- Liver function tests: Assess hepatic involvement and distinguish injury patterns 1, 5
Clinical Assessment for Compartment Syndrome
Pain, tension, paresthesia, and paresis are early warning signs of compartment syndrome, which can both cause and complicate rhabdomyolysis. 1 Late signs include pulselessness and pallor, which typically indicate irreversible damage. 1 Consider fasciotomy when compartment pressure exceeds 30 mmHg or when the differential pressure (diastolic blood pressure minus compartment pressure) is <30 mmHg. 1
Monitoring Strategy
Initial Phase (Severe Cases with CK >5,000 IU/L):
- Monitor CK, electrolytes, and renal function every 6-12 hours initially 5
- Monitor potassium closely as hyperkalemia can cause fatal cardiac arrhythmias 1
- Perform serial CK measurements to track disease progression 5
Recovery Phase:
- Once CK levels begin decreasing, reduce monitoring frequency to daily 5
- Continue monitoring until CK falls below 1,000 U/L 3
Etiology-Specific Testing
When the cause is unclear or in cases of recurrent rhabdomyolysis:
- Medication review: Identify causative agents, particularly statins (incidence 1.6 per 100,000 patient-years) 1
- Viral studies: If viral myositis is suspected 1, 5
- Autoimmune markers (ANA, ASMA, ANCA): If autoimmune myositis is suspected 1, 5
- Genetic testing: Consider SLCO1B1 gene mutations for statin-induced cases 1
- Malignant hyperthermia susceptibility testing: For recurrent rhabdomyolysis, exertional rhabdomyolysis, or persistently elevated CK without identified cause 5
Critical Pitfalls to Avoid
The absence of the classic triad (myalgia, weakness, pigmenturia) does not exclude rhabdomyolysis—maintain high clinical suspicion with any unexplained CK elevation. 2 Do not wait for myoglobinuria to appear, as it may be absent or transient due to myoglobin's short half-life. 5 In disaster or trauma settings with crush injuries, CK levels >75,000 IU/L indicate >80% risk of acute kidney injury requiring immediate aggressive intervention. 5