What laboratory tests and management strategies are recommended for rhabdomyolysis?

Laboratory Tests for Rhabdomyolysis

The essential initial laboratory workup for suspected rhabdomyolysis includes serum creatine kinase (CK), complete metabolic panel with electrolytes (particularly potassium), serum creatinine, and urinalysis for myoglobinuria. 1, 2, 3

Core Diagnostic Tests

Primary Diagnostic Markers

• Serum creatine kinase (CK) is the most sensitive laboratory finding for muscle injury, with levels >1,000 U/L or at least 5 times the upper limit of normal (>975 IU/L) confirming the diagnosis 1, 3, 4
• Serum myoglobin should be measured as it is released earlier than CK from damaged muscle tissue, though it has a shorter half-life 1, 2
• Urinalysis showing brown color, cloudiness, and positive for blood without RBCs indicates myoglobinuria, though qualitative urine myoglobin by dipstick is positive in only 19-40% of cases and is not a reliable screening tool 1, 4, 5

Essential Electrolyte and Metabolic Panel

• Complete electrolyte panel including potassium, calcium, phosphorus, and magnesium is crucial, as hyperkalemia can lead to life-threatening cardiac arrhythmias 1, 2
• Serum creatinine and BUN to monitor for acute kidney injury, which occurs in 15-46% of rhabdomyolysis cases 2, 3, 4
• Arterial blood gas analysis to assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis 1, 2

Additional Initial Tests

• Complete blood count with differential to evaluate for infection or other systemic processes 1
• Liver function tests (AST, ALT, alkaline phosphatase) to assess for transaminitis from muscle enzyme release and distinguish hepatocellular patterns 1, 2
• Coagulation studies to evaluate for disseminated intravascular coagulation in severe cases 1, 2
• ECG and cardiac troponin to rule out cardiac involvement, especially in severe rhabdomyolysis 1, 2

Serial Monitoring During Treatment

Monitoring Frequency Based on Severity

• For severe rhabdomyolysis (CK >5,000-15,000 IU/L): Monitor CK, electrolytes, and renal function every 6-12 hours initially 2
• Once CK levels begin to decrease: Reduce monitoring frequency to daily 2
• Continue IV fluids until CK levels drop below 1,000 U/L 6

Critical Parameters to Track

• Repeated plasma myoglobin, CK, and potassium measurements throughout treatment 1
• Electrolytes, particularly potassium, to monitor for life-threatening hyperkalemia 1
• Renal function tests to detect acute kidney injury early 2

Etiology-Specific Testing

When Specific Causes Are Suspected

• Medication review to identify causative agents, particularly statins, which cause rhabdomyolysis at an incidence of 1.6 per 100,000 patient-years 1
• Viral studies if viral myositis is suspected 1, 2
• Autoimmune markers (ANA, ASMA, ANCA) if autoimmune myositis is suspected 1, 2

For Metabolic or Hereditary Disorders

• Serum calcium, phosphorus, alkaline phosphatase, parathyroid hormone, and 25-hydroxy-vitamin D if metabolic disorder is suspected 1
• Genetic testing for SLCO1B1 gene mutations in statin-induced cases 1
• RYR1 and CACNA1S gene sequencing for malignant hyperthermia susceptibility in exertional rhabdomyolysis or heat illness 1
• CPT2, PYGM, ACADM, AMPD1, and VLCAD gene testing for metabolic myopathies causing recurrent rhabdomyolysis 1

Referral Criteria for Specialized Testing

• Consider referral for malignant hyperthermia investigation in cases of exertional rhabdomyolysis, recurrent episodes, persistently elevated CK after full neurological evaluation, or family history of neuromuscular disorders 1, 2

Special Clinical Considerations

Compartment Syndrome Assessment

• Monitor for early signs: pain, tension, paresthesia, and paresis 1, 2
• Late signs (pulselessness and pallor) indicate irreversible damage 1
• Consider fasciotomy when compartment pressure exceeds 30 mmHg or when differential pressure (diastolic BP – compartment pressure) is <30 mmHg 1

Trauma-Specific Considerations

• In traumatic rhabdomyolysis from falls: CK levels peak 24-120 hours after the event due to lymphatic clearance mechanisms; repeat CK measurement at 24 hours if clinical suspicion remains high 1
• In crush injury patients: Point-of-care devices like iStat can provide rapid assessment of creatinine and potassium when standard laboratory infrastructure is limited 2
• CK levels above 75,000 IU/L in crush syndrome are associated with >80% risk of acute kidney injury 2

Risk Stratification

• McMahon score of 6 or greater calculated on admission predicts acute kidney injury requiring renal replacement therapy 3