What lab orders should be given to a patient for initial evaluation?

Laboratory Orders for Initial Evaluation of Rhabdomyolysis

For a patient with suspected rhabdomyolysis, immediately order: CBC with differential, comprehensive metabolic panel (including electrolytes, calcium, magnesium, phosphorus, BUN, creatinine, liver function tests), creatine kinase (CK), serum myoglobin, urinalysis, coagulation studies, and arterial blood gas for lactate and base excess. 1

Core Essential Laboratory Tests

The initial laboratory workup must be comprehensive to assess both the diagnosis and potential complications:

• Complete Blood Count (CBC) with differential to evaluate for infection, anemia from hemolysis, or other systemic processes 2, 1

• Comprehensive Metabolic Panel (CMP) including:

  • Serum electrolytes (sodium, potassium, chloride, bicarbonate) 2, 1
  • Calcium, magnesium, and phosphorus levels (hyperkalemia and hypocalcemia are life-threatening complications) 1
  • Blood urea nitrogen (BUN) and creatinine to assess renal function and acute kidney injury risk 2, 1
  • Liver function tests (AST, ALT, alkaline phosphatase, bilirubin) as transaminitis commonly occurs due to muscle enzyme release 1

• Creatine Kinase (CK) is the primary diagnostic marker; levels >1,000 IU/L suggest rhabdomyolysis, with severe cases showing CK >15,000 IU/L 1

• Serum myoglobin for early detection, though CK is more practical for serial monitoring 1

• Urinalysis to detect myoglobinuria (brown/tea-colored urine, positive for blood without RBCs on microscopy) 1

Critical Monitoring Parameters

Potassium levels require immediate attention as hyperkalemia can precipitate life-threatening cardiac arrhythmias and must be corrected emergently 1. Serial electrolyte measurements, particularly potassium, should be monitored throughout treatment 1.

• Coagulation studies (PT, PTT, fibrinogen, D-dimer) to evaluate for disseminated intravascular coagulation, a potential complication of severe rhabdomyolysis 2, 1
• Arterial blood gas analysis to assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis 1

Additional Muscle Enzymes

Beyond CK, measure additional muscle enzymes that are commonly elevated:

• LDH, AST (SGOT), ALT (SGPT), and aldolase provide supporting evidence of muscle breakdown 1

Etiology-Specific Testing

Depending on clinical context, consider:

• Viral studies if viral myositis is suspected 1
• Autoimmune markers (ANA, ASMA, ANCA) if autoimmune myositis is in the differential 1
• Cardiac assessment including ECG and cardiac troponin to rule out cardiac involvement in severe cases 1
• Genetic testing (RYR1, CACNA1S, CPT2, PYGM, ACADM, AMPD1, VLCAD) for patients with recurrent rhabdomyolysis, exercise intolerance, or family history of neuromuscular disorders 1
• Metabolic workup including serum calcium, phosphorus, alkaline phosphatase, parathyroid hormone, and 25-hydroxy-vitamin D if metabolic or hereditary disorder is suspected 1

Serial Monitoring Strategy

Repeat CK, creatinine, and electrolytes daily until CK is declining and renal function is stable 1. The timing of CK measurement is critical: CK peaks between 24-120 hours after the inciting event, not immediately 1. If initial CK at 9 hours post-event is only mildly elevated but clinical suspicion remains high, repeat measurement at 24 hours is essential 1.

Common Pitfalls to Avoid

• Do not rely on a single CK measurement if obtained early (within 9 hours) after the inciting event, as levels may still be rising 1
• Impact trauma from falls can elevate CK without true rhabdomyolysis; repeat measurement at 24 hours helps distinguish simple contusion from true muscle breakdown 1
• Do not delay fluid resuscitation while awaiting complete laboratory results; early intervention is critical to prevent acute kidney injury 1