Laboratory Workup for Rhabdomyolysis
The essential laboratory workup for suspected rhabdomyolysis should include serum creatine kinase (CK), plasma myoglobin, complete electrolyte panel (with particular attention to potassium), renal function tests, and urinalysis. 1, 2
Core Laboratory Tests
Serum creatine kinase (CK): The primary diagnostic marker with levels typically >1000 U/L or at least 5 times the upper limit of normal. CK levels should be monitored serially to track disease progression and response to treatment. 2, 3
Serum myoglobin: Important to measure as myoglobin is directly nephrotoxic and responsible for acute kidney injury in rhabdomyolysis. 1, 2
Complete electrolyte panel: Particularly important to monitor potassium, as hyperkalemia can lead to life-threatening cardiac arrhythmias. Also check calcium, phosphorus, and magnesium levels. 1, 4
Renal function tests: Serum creatinine and blood urea nitrogen (BUN) to assess for acute kidney injury, which occurs in 15-33% of rhabdomyolysis cases. 2, 3
Urinalysis: To check for myoglobinuria (which may appear as heme-positive with few or no red blood cells on microscopy). Note that urine myoglobin testing by dipstick/ultrafiltration may be positive in only 19% of cases. 2, 3
Additional Tests Based on Clinical Context
Liver function tests: To assess for hepatic involvement and distinguish between hepatocellular and other patterns of injury. 4
Creatine kinase isoenzymes: May help differentiate between cardiac and skeletal muscle injury. 1
Coagulation studies: To evaluate for disseminated intravascular coagulation, which can be a complication of severe rhabdomyolysis. 4
Arterial blood gas: To assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis. 4
Etiology-Specific Testing
Toxicology screen: For patients with suspected drug or toxin-induced rhabdomyolysis (responsible for up to 46% of cases). 3, 5
Viral studies: Consider testing for viral myositis if clinically indicated. 4
Autoimmune markers: Consider ANA, ASMA, ANCA if autoimmune myositis is suspected. 4
Compartment pressure measurement: If compartment syndrome is suspected based on clinical findings of pain, tension, paresthesia, or paresis in affected limbs. 1
Special Considerations
Repeat CK measurements: Serial monitoring is essential to track the course of muscle breakdown and guide fluid therapy. 1, 2
Note on false negatives: Rarely, rhabdomyolysis can present with normal CK levels, particularly if testing is delayed after the initial muscle injury. In these cases, urine or serum myoglobin may be more reliable. 6
Consider muscle biopsy: In cases of recurrent rhabdomyolysis, family history of muscle disorders, or when standard treatment fails to resolve symptoms, as approximately 10% of cases may be due to underlying myopathies. 3, 7
Cardiac assessment: Consider ECG and cardiac troponin in patients with severe rhabdomyolysis to rule out cardiac involvement. 4
Monitoring During Treatment
Fluid balance: Monitor input/output to ensure adequate hydration and urine output. 1, 2
Electrolytes: Check at least twice daily initially, with particular attention to potassium, calcium, and phosphorus. 1, 5
Renal function: Monitor creatinine and BUN at least daily to assess for development or progression of acute kidney injury. 2
Urine pH: Consider monitoring if alkalinization strategy is employed, though evidence for this approach is limited. 2
By following this comprehensive laboratory workup, clinicians can effectively diagnose rhabdomyolysis, identify its cause, monitor for complications, and guide appropriate treatment decisions.