What laboratory tests should be ordered to diagnose rhabdomyolysis (muscle breakdown) and pericarditis (inflammation of the heart sac)?

Laboratory Tests for Diagnosing Rhabdomyolysis and Pericarditis

For diagnosing rhabdomyolysis and pericarditis, specific laboratory tests are required including creatine kinase (CK), myoglobin, and troponins for rhabdomyolysis, and inflammatory markers, cardiac enzymes, and complete blood count for pericarditis.

Diagnostic Tests for Rhabdomyolysis

First-line Laboratory Tests

• Creatine kinase (CK) - the primary diagnostic marker with levels typically at least 5-10 times the upper limit of normal 1
• Serum myoglobin - may be elevated before CK 1
• Complete blood count - to assess for infection or inflammation 1
• Renal function tests (BUN, creatinine) - to evaluate kidney injury 1
• Electrolytes (potassium, calcium, phosphorus) - to monitor for hyperkalemia and hypocalcemia 2
• Liver function tests - AST and ALT may be elevated and can serve as alternative markers 3
• Urinalysis - may show myoglobinuria (positive for blood on dipstick with few or no RBCs on microscopy) 4

Important Considerations

• CK levels typically peak 24-36 hours after muscle injury and decline at a rate of approximately 40% per day 1
• AST ≥110 U/L has shown 97.1% sensitivity for detecting CK ≥5,000 U/L when CK testing is unavailable 3
• Normal CK does not always exclude rhabdomyolysis, especially if testing is delayed after peak muscle injury 4

Diagnostic Tests for Pericarditis

First-line Laboratory Tests

• C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) - markers of inflammation 5
• Complete blood count with differential - to assess for infection or inflammation 5
• Cardiac biomarkers - troponins and CK to detect myocardial involvement 5
• Renal function tests - to evaluate kidney function 5
• Liver function tests - to assess for systemic involvement 5
• Thyroid function tests - to rule out thyroid-related causes 5

Additional Tests Based on Clinical Presentation

• For suspected autoimmune pericarditis: ANA, ENA, ANCA, ferritin (if Still's disease suspected) 5
• For suspected tuberculous pericarditis: IGRA test (Quantiferon, ELISpot) 5
• For suspected viral pericarditis: PCR for viral genomes, serology for HCV and HIV 5
• For suspected bacterial pericarditis: Blood cultures before antibiotic administration 5

Diagnostic Algorithm

For Rhabdomyolysis:

1. Order initial laboratory panel:

   • CK (primary marker)
   • Serum myoglobin
   • Renal function tests
   • Electrolytes
   • Liver function tests
   • Urinalysis

2. Interpret results:

   • CK ≥5,000 U/L indicates clinically significant rhabdomyolysis with risk of renal injury 2
   • If CK testing is unavailable, AST ≥110 U/L can be used as a surrogate marker 3
   • Monitor for electrolyte abnormalities, especially hyperkalemia and hypocalcemia 2

For Pericarditis:

1. Order initial laboratory panel:

   • CRP and ESR
   • Complete blood count with differential
   • Cardiac biomarkers (troponins, CK)
   • Renal and liver function tests
   • Thyroid function tests 5

2. Additional testing based on clinical features:

   • If fever >38°C, subacute course, large effusion, or tamponade: perform comprehensive etiologic workup 5
   • If suspected specific etiology (autoimmune, infectious, etc.): order targeted tests as outlined above 5

Pitfalls and Caveats

• For rhabdomyolysis:

  • CK may be normal in some cases of rhabdomyolysis if measured after the peak or very early in the course 4
  • Myoglobinuria may be missed if urinalysis is delayed due to rapid renal clearance of myoglobin 1
  • Elevated liver enzymes (especially AST) may be misinterpreted as primary liver disease rather than muscle breakdown 3

• For pericarditis:

  • Cardiac biomarkers may be elevated due to associated myocarditis rather than primary pericardial disease 5
  • Normal inflammatory markers do not exclude pericarditis, especially if the patient is already on anti-inflammatory treatment 5
  • Specific etiologic diagnosis often requires additional specialized testing beyond the initial laboratory workup 5