What labs are recommended for the diagnosis and management of rhabdomyolysis?

Laboratory Workup for Rhabdomyolysis

For diagnosis and management of rhabdomyolysis, obtain creatine kinase (CK), complete metabolic panel with electrolytes (particularly potassium, calcium, phosphorus, magnesium), serum creatinine, urinalysis for myoglobinuria, complete blood count, liver function tests, and consider serum myoglobin. 1, 2

Essential Initial Laboratory Tests

Core Diagnostic Tests

• Creatine kinase (CK) is the primary diagnostic marker, with diagnosis confirmed when CK exceeds 1,000 U/L or is at least 5 times the upper limit of normal 2, 3
• Serum myoglobin should be measured, though plasma myoglobin requires repeated bioassessment for monitoring 1
• Urinalysis is critical to detect myoglobinuria, which appears as brown-colored urine that is positive for blood without red blood cells 1, 2
  • Note that urine myoglobin detected by dipstick is positive in only 19% of cases, so negative results do not exclude rhabdomyolysis 4

Complete Metabolic Panel

• Serum electrolytes including sodium, potassium, calcium, phosphorus, and magnesium must be measured, as hyperkalemia can cause life-threatening cardiac arrhythmias 1
• Blood urea nitrogen and serum creatinine are essential to assess for acute kidney injury, which occurs in 15-33% of rhabdomyolysis cases 2, 4
• Glucose levels should be monitored during treatment 1

Additional Standard Tests

• Complete blood count with differential to evaluate for infection or systemic processes 1
• Liver function tests (AST, ALT, alkaline phosphatase) are necessary because transaminitis commonly occurs due to muscle enzyme release 1
• Coagulation studies should be performed to evaluate for disseminated intravascular coagulation in severe cases 1

Monitoring During Treatment

• Repeated electrolyte measurements, particularly potassium, are critical to monitor for life-threatening hyperkalemia 1
• Serial CK measurements should be obtained, as CK levels peak 24-120 hours after the initial insult due to lymphatic clearance mechanisms 1
• Arterial blood gas analysis is essential in severe cases to assess for metabolic acidosis 1

Etiology-Specific Testing

When Metabolic or Hereditary Disorder is Suspected

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

When Autoimmune Myositis is Suspected

• Autoimmune markers including ANA, ASMA, and ANCA should be evaluated 1

When Infectious Etiology is Suspected

• Viral studies for viral myositis should be considered if clinically indicated 1

Cardiac Assessment

• ECG and cardiac troponin should be performed in patients with severe rhabdomyolysis to rule out cardiac involvement 1

Common Pitfalls

• Do not rely solely on urine myoglobin dipstick testing, as it has low sensitivity (positive in only 19% of cases) 4
• CPK testing is not routinely needed at baseline but should be performed if myalgia develops during treatment 5
• If CK remains persistently elevated (idiopathic hyperCKemia) after full neurological evaluation, further investigation is required 1
• Repeat CK measurement at 24 hours post-injury if clinical suspicion remains high, as CK levels may be delayed due to lymphatic clearance 1

Risk Stratification

• A McMahon score of 6 or greater calculated on admission predicts acute kidney injury requiring renal replacement therapy 2
• CK levels >15,000-16,000 IU/L indicate severe rhabdomyolysis with higher risk of acute kidney injury and may require fluid resuscitation volumes exceeding 6L 1, 3