What laboratory tests should be ordered for suspected rhabdomyolysis?

Laboratory Workup for Suspected Rhabdomyolysis

Order creatine kinase (CK), comprehensive metabolic panel with electrolytes (including calcium, magnesium, phosphorus, and potassium), urinalysis, and serum myoglobin as your initial laboratory assessment for suspected rhabdomyolysis. 1, 2

Essential Initial Laboratory Tests

Core Diagnostic Tests

• Serum creatine kinase (CK): This is the primary diagnostic marker; levels >1,000 U/L or at least 5 times the upper limit of normal confirm rhabdomyolysis 2, 3
• Serum myoglobin: Obtain plasma myoglobin levels as part of repeated bioassessment 1
• Urinalysis: Check specifically for myoglobinuria (brown/cloudy urine that is positive for blood without RBCs on microscopy) 1, 2

Critical Electrolyte and Renal Function Panel

• Complete metabolic panel including:
  • Serum creatinine and blood urea nitrogen to assess for acute kidney injury 1, 2
  • Potassium (critical - hyperkalemia can cause life-threatening cardiac arrhythmias) 1
  • Calcium, phosphorus, and magnesium 1
  • Glucose 4

Monitor electrolytes closely and serially, particularly potassium, as this is the most immediately life-threatening complication. 1

Additional Essential Tests

• Complete blood count with differential to evaluate for infection or other systemic processes 4
• Liver function tests (AST, ALT, alkaline phosphatase): Transaminitis commonly occurs in rhabdomyolysis due to muscle enzyme release 1, 5
• Arterial blood gas: Essential to assess for metabolic acidosis, which commonly accompanies severe rhabdomyolysis 1

Cardiac Monitoring

• Electrocardiogram: Obtain immediately to assess for hyperkalemia-induced cardiac changes 1
• Cardiac troponin: Perform in severe cases to rule out cardiac muscle involvement 1

Etiology-Specific Testing (Based on Clinical Context)

If Infection Suspected

• Blood, urine, and throat cultures 4
• Viral studies if viral myositis is suspected 1

If Autoimmune Myositis Suspected

• Autoimmune markers: ANA, ASMA, ANCA 1

If Drug-Induced (Particularly Statins)

• Review all medications and supplements 1
• Consider genetic testing for SLCO1B1 gene mutations in statin-induced cases 1

If Metabolic/Hereditary Disorder Suspected (Recurrent Episodes)

• Serum calcium, phosphorus, alkaline phosphatase 4
• Parathyroid hormone and 25-hydroxy-vitamin D 4
• Serum copper and ceruloplasmin 4
• Consider fibroblast culture and/or DNA analysis if osteogenesis imperfecta suspected 4

If Coagulopathy Suspected

• Coagulation studies (PT, PTT, INR, fibrinogen) to evaluate for disseminated intravascular coagulation 1
• Platelet count 4

Serial Monitoring During Treatment

Trend CK, creatinine, and electrolytes daily until CK is declining and renal function is stable. 1, 5

• Repeat CK levels to monitor response to treatment 1
• Serial electrolytes, particularly potassium 1
• Daily serum creatinine to monitor for acute kidney injury development 1, 2

Important Clinical Caveats

• Urine myoglobin by dipstick is only positive in 19% of cases, so its absence does not exclude rhabdomyolysis 6
• CK levels can reach extremely high values (>100,000 IU/L or even 590,000 IU/L) without necessarily causing acute kidney injury 5, 3
• Acute kidney injury risk is particularly high when CK >15,000-16,000 IU/L 1, 3
• The classic triad of myalgia, weakness, and pigmenturia is seen in less than 10% of patients 3
• Patients may be normothermic or hypothermic despite infection as a precipitating cause 4