Workup for Rhabdomyolysis
For suspected rhabdomyolysis, immediately obtain serum creatine kinase (CK), serum creatinine, electrolytes (particularly potassium), and urinalysis for myoglobinuria 1.
Initial Laboratory Assessment
The core diagnostic workup requires:
Serum CK levels: This is the primary diagnostic marker. CK >10,000 IU/L confirms exertional rhabdomyolysis, while >5,000 IU/L indicates non-exertional rhabdomyolysis 2. In patients with underlying neuromuscular conditions, CK should exceed 5-10 times their baseline 2.
Serum creatinine: Essential for detecting acute kidney injury, the most serious complication. Risk is particularly high when CK exceeds 15,000-16,000 IU/L 3, 4.
Electrolytes with focus on potassium: Hyperkalemia can cause life-threatening cardiac arrhythmias and requires immediate ECG evaluation 3, 5.
Urinalysis for myoglobinuria: Myoglobin causes the characteristic dark "tea-colored" urine and indicates risk for renal tubular obstruction 1.
Plasma myoglobin: May be more sensitive than CK for early detection of kidney injury risk, as it peaks earlier than CK 3.
Additional Essential Tests
Beyond the core panel, obtain:
Liver transaminases (AST/ALT): These are often elevated due to muscle breakdown and help assess overall tissue injury 6.
Aldolase and LDH: Provide additional markers of muscle inflammation 6.
Troponin and ECG: Critical to evaluate for myocardial involvement, particularly in severe cases 6.
Urine output monitoring: Place a bladder catheter to monitor hourly output and urine pH, which should be maintained ≥6.5 3.
Risk Stratification
Use the McMahon score to predict acute kidney injury risk and guide treatment intensity 2, 7. This determines whether aggressive fluid resuscitation or dialysis may be needed.
When to Investigate Underlying Causes
If this is the first episode with a clear precipitant (trauma, drugs, prolonged immobilization), extensive workup for inherited causes is unnecessary 4. However, further investigation is warranted if:
- Recurrent episodes of rhabdomyolysis occur
- History of exercise intolerance exists
- Positive family history for neuromuscular disorders
- Persistently elevated baseline CK (idiopathic hyperCKemia) 8
- Episode occurs with minimal exertion or no clear trigger
Workup for Recurrent/Unexplained Cases
When inherited causes are suspected, evaluate for:
Metabolic disorders: Screen for hypothyroidism (TSH), vitamin D deficiency, and other endocrine abnormalities that increase muscle symptom risk 1.
Autoimmune conditions: Check inflammatory markers (ESR, CRP) and consider rheumatologic disorders like polymyalgia rheumatica 1.
Genetic testing: DNA analysis for disorders of glycogen metabolism, fatty acid β-oxidation, mitochondrial oxidative phosphorylation, and RYR1 variants (malignant hyperthermia susceptibility) 8, 9. Note that genetic testing identifies a definitive cause in only a minority of cases, with many variants of unknown significance 10.
Muscle biopsy: Consider when diagnosis remains uncertain after non-invasive testing, though many biopsies show non-specific changes 9, 10.
EMG and muscle imaging (MRI): Useful when overlap with neurologic syndromes is suspected 6.
Critical Timing Considerations
CK levels should peak 1-4 days after the inciting event and normalize to baseline within 1-2 weeks of rest 2. If CK remains elevated beyond this timeframe, suspect an underlying myopathy rather than simple rhabdomyolysis.
Common Pitfalls to Avoid
- Don't wait for the classic triad of myalgia, weakness, and dark urine—this occurs in <10% of patients 4.
- Don't overlook medication interactions: the combination of statins with certain macrolides (clarithromycin, erythromycin) dramatically increases rhabdomyolysis risk 11.
- Don't forget to evaluate for compartment syndrome in trauma patients with rhabdomyolysis—this requires urgent fasciotomy 3.
- Don't discontinue statins unnecessarily in mild cases; establish baseline muscle symptoms before starting therapy to avoid confusion 1.