What are the diagnostic tests and management for suspected rhabdomyolysis (muscle breakdown disease)?

Diagnostic Testing for Rhabdomyolysis

The essential diagnostic test for rhabdomyolysis is serum creatine kinase (CK), with a diagnostic threshold of at least 5 times the upper limit of normal (>975 IU/L), though levels ≥10 times normal with rapid decline provide more definitive confirmation. 1, 2, 3

Core Laboratory Panel

Initial blood work must include:

• Serum CK - the primary diagnostic marker, with levels >5,000 IU/L indicating severe disease and >15,000-16,000 IU/L associated with high risk of acute kidney injury 1, 4, 2, 3
• Serum myoglobin - released earlier than CK but has shorter half-life, useful for early detection 1, 4
• Complete metabolic panel including creatinine and BUN to assess renal function and detect acute kidney injury 1, 4
• Electrolytes - specifically potassium (hyperkalemia risk), calcium (hypocalcemia), and phosphorus (hyperphosphatemia) 1, 4
• Liver function tests (AST, ALT, LDH) - transaminases are elevated due to muscle enzyme release, not hepatic injury 5, 1, 4

Urinalysis Findings

• Dipstick positive for blood without RBCs on microscopy indicates myoglobinuria, though this is only positive in 19% of cases 5, 4, 6
• Brown or tea-colored urine suggests myoglobinuria 4, 2

Critical caveat: Normal urine myoglobin does NOT exclude rhabdomyolysis - one case series found myoglobin positive in only 19% despite confirmed disease 6. Additionally, severe rhabdomyolysis with acute kidney injury can occur with normal CK at presentation 7.

Additional Testing Based on Severity

For severe cases (CK >15,000 IU/L or clinical concern):

• Cardiac troponin and ECG - to assess for myocardial involvement and detect arrhythmias from hyperkalemia 5, 1, 4
• Coagulation studies (PT, PTT, fibrinogen) - to evaluate for disseminated intravascular coagulation 1, 4
• Arterial blood gas - to assess for metabolic acidosis 1, 4

Monitoring Frequency

• Severe rhabdomyolysis (CK >5,000 IU/L): Monitor CK, electrolytes, and renal function every 6-12 hours initially 1
• Once CK begins declining: Reduce to daily monitoring 1
• Serial CK measurements are essential to track disease course 1, 3

Etiology-Specific Testing

Consider additional workup when indicated:

• Viral studies if viral myositis suspected 1, 4
• Autoimmune markers (ANA, ASMA, ANCA) if autoimmune myositis suspected, particularly in immune checkpoint inhibitor therapy patients 5, 1, 4
• EMG and MRI may help differentiate inflammatory myositis from rhabdomyolysis 5, 1
• Genetic testing (RYR1, CACNA1S, CPT2, PYGM) for recurrent rhabdomyolysis, exercise intolerance, or family history of neuromuscular disorders 4, 3

Special Considerations for Trauma Cases

• Compartment syndrome assessment - monitor for pain, tension, paresthesia, and paresis (early signs); pulselessness and pallor indicate late/irreversible damage 4, 8
• Point-of-care testing (iStat) can provide rapid creatinine and potassium in disaster/crush injury settings 1
• CK levels >75,000 IU/L in crush syndrome carry >80% risk of acute kidney injury 1

Important timing consideration: In traumatic rhabdomyolysis from falls, CK peaks 24-120 hours post-injury due to lymphatic clearance mechanisms - repeat CK at 24 hours if initial level normal but clinical suspicion remains 4

When to Suspect Underlying Metabolic Disease

Refer for malignant hyperthermia susceptibility testing if: 1, 4

• Exertional rhabdomyolysis
• Recurrent episodes
• Persistently elevated CK (idiopathic hyperCKemia) after neurological workup
• No identified cause after standard evaluation