What labs should be checked to diagnose rhabdomyolysis (muscle breakdown)?

Laboratory Testing for Rhabdomyolysis

The essential initial laboratory tests for diagnosing rhabdomyolysis include serum creatine kinase (CK), complete metabolic panel with electrolytes (particularly potassium, calcium, and phosphorus), renal function tests (creatinine and BUN), and urinalysis to detect myoglobinuria. 1, 2

Core Diagnostic Tests

Primary Muscle Injury Markers

• Creatine kinase (CK) is the primary diagnostic marker, with levels ≥10 times the upper limit of normal (typically >1,000 U/L) considered diagnostic for rhabdomyolysis 3, 4
• Serum myoglobin can be detected earlier than CK but has a shorter half-life, making it less reliable for diagnosis 2
• Additional muscle enzymes including LDH, AST (SGOT), ALT (SGPT), and aldolase should be measured as they are commonly elevated 5, 1

Urinalysis

• Urinalysis showing brown/tea-colored urine that is positive for blood without red blood cells indicates myoglobinuria, a hallmark finding 1
• Note that urine myoglobin detected by dipstick is positive in only 19% of cases, so a negative test does not exclude rhabdomyolysis 6

Essential Electrolyte and Metabolic Panel

Critical Electrolytes

• Complete electrolyte panel must include potassium, calcium, phosphorus, and magnesium 1, 2
• Hyperkalemia is life-threatening and can precipitate cardiac arrhythmias requiring emergent correction 1
• Hypocalcemia and hyperphosphatemia are common complications that must be identified 2

Renal Function Assessment

• Serum creatinine and BUN to monitor for acute kidney injury, the most serious complication occurring in 15-46% of cases 2, 3, 6
• Acute kidney injury risk is particularly high when CK exceeds 16,000 IU/L 3

Additional Laboratory Tests Based on Clinical Context

Hepatic and Coagulation Assessment

• Liver function tests (AST, ALT, alkaline phosphatase) to assess for hepatic involvement and distinguish transaminitis from muscle enzyme release 1, 2
• Coagulation studies to evaluate for disseminated intravascular coagulation in severe cases 1, 2

Acid-Base Status

• Arterial blood gas analysis to assess for metabolic acidosis, which commonly occurs in severe rhabdomyolysis 1, 2

Cardiac Assessment

• ECG and cardiac troponin should be performed in severe cases to rule out cardiac involvement 2
• ECG monitoring is crucial to detect arrhythmias from electrolyte abnormalities 2

Etiology-Specific Testing

Infectious and Autoimmune Workup

• Viral studies if viral myositis is suspected 1, 2
• Autoimmune markers (ANA, ASMA, ANCA) if autoimmune myositis is considered 1, 2

Medication and Toxin Screen

• Complete medication review including statins, antipsychotics, and over-the-counter supplements 1
• Toxicology screen for illicit drugs (cocaine, methamphetamine, MDMA) and alcohol, as exogenous toxins cause 46% of cases 6

Genetic and Metabolic Testing (for Recurrent Cases)

• Genetic testing for RYR1 and CACNA1S (malignant hyperthermia susceptibility) in patients with exertional rhabdomyolysis 1, 2
• CPT2, PYGM, ACADM, AMPD1, and VLCAD gene testing for metabolic myopathies in recurrent cases 1
• Metabolic workup including calcium, phosphorus, parathyroid hormone, and 25-hydroxy-vitamin D if hereditary disorder suspected 1

Serial Monitoring During Treatment

Frequency of Monitoring

• For severe rhabdomyolysis (CK >5,000 IU/L): Monitor CK, electrolytes, and renal function every 6-12 hours initially 2
• Once CK begins declining: Reduce monitoring to daily 2
• Continue monitoring until: CK falls below 1,000 U/L and renal function stabilizes 1, 4

Key Monitoring Parameters

• Serial CK measurements to track disease progression and response to treatment 2
• Daily electrolytes, particularly potassium, to prevent life-threatening arrhythmias 1
• Trending creatinine to detect acute kidney injury early 1

Important Clinical Pitfalls

• Do not rely on urine myoglobin alone for diagnosis, as it is positive in less than 20% of cases despite true rhabdomyolysis 6
• CK levels peak 24-120 hours after muscle injury, not immediately, so repeat testing at 24 hours if initial levels are only mildly elevated but clinical suspicion remains high 1
• In trauma patients, simple contusion can elevate CK without true rhabdomyolysis, requiring clinical correlation 1
• Assess for compartment syndrome in traumatic cases with clinical signs of pain, tension, paresthesia, and paresis, as this requires urgent fasciotomy 1, 2