Can You Order CK/CPK for Suspected Rhabdomyolysis?
Yes, you should absolutely order a CK/CPK test for suspected rhabdomyolysis—it is the preferred diagnostic marker and should be obtained immediately at presentation, with serial measurements to monitor disease progression and guide treatment decisions. 1
Diagnostic Approach
Initial Testing
- Order total CK (not CK-MB) as the primary diagnostic test for rhabdomyolysis, as CK-MB lacks the sensitivity and specificity needed for rhabdomyolysis assessment and is only useful for cardiac injury evaluation 1
- Obtain CK at hospital presentation, then repeat at 6-9 hours and serially thereafter, as CK levels peak 24-120 hours after the inciting event and may still be rising at initial presentation 2, 1
- A CK level ≥5 times the upper limit of normal (typically >1,000 U/L) is the diagnostic threshold for rhabdomyolysis, though levels >15,000 U/L indicate severe disease requiring aggressive fluid resuscitation 1, 3
Essential Concurrent Laboratory Tests
Beyond CK, you must order:
- Complete metabolic panel including potassium, calcium, phosphorus, and magnesium—hyperkalemia can precipitate life-threatening cardiac arrhythmias and requires emergent correction 1
- Serum creatinine to assess for acute kidney injury, which is the most serious complication of rhabdomyolysis 1
- Urinalysis looking for brown/cloudy urine that is positive for blood without RBCs (indicating myoglobinuria), which is a key diagnostic finding 1, 4
- Complete blood count with differential to evaluate for infection or systemic processes 1
- Liver function tests (AST, ALT) as these are commonly elevated due to muscle enzyme release, though AST ≥110 U/L can serve as an alternative marker when CK is unavailable 1, 3
Critical Diagnostic Pitfalls
Normal CK Does Not Exclude Rhabdomyolysis
- Be aware that CK can be normal at presentation in confirmed rhabdomyolysis cases, particularly if the patient presents very early or if myoglobin has already been cleared 4
- If clinical suspicion remains high despite normal initial CK, repeat testing at 24 hours is essential, as the lymphatic clearance mechanism causes delayed CK elevation 1
- In rare cases, severe rhabdomyolysis with acute kidney injury can occur with persistently normal CK—kidney biopsy showing myoglobin casts may be needed for diagnosis 4
Timing Considerations
- At 9 hours post-exercise or injury, CK levels are typically still rising and have not reached their maximum—do not assume peak values have been reached 1
- Impact trauma from falls can drastically elevate CK without reflecting true muscle breakdown, but given clinical context, assume true rhabdomyolysis until proven otherwise 1
Monitoring Strategy
Serial CK Measurements
- Trend CK, creatinine, and electrolytes daily until CK is declining and renal function is stable 1
- For severe rhabdomyolysis (CK >15,000 U/L), initiate aggressive fluid resuscitation with volumes >6L per day; for moderate cases (CK 3,000-15,000 U/L), 3-6L per day is typically sufficient 1
Additional Monitoring Parameters
- Monitor urine output closely—adequate hydration is critical to prevent myoglobin-induced renal tubular obstruction 1
- Perform ECG monitoring for cardiac arrhythmias related to hyperkalemia 1
- Consider CPK testing if myalgia develops during treatment, though routine CPK monitoring is not necessary in asymptomatic patients 2
Etiology-Specific Testing
When rhabdomyolysis is confirmed, investigate the underlying cause:
- Medication review: Document all prescription medications (especially statins), over-the-counter supplements (red yeast rice, creatine monohydrate, wormwood oil, licorice, Hydroxycut), and recreational drugs (cocaine, methamphetamine, MDMA) 1
- Thyroid function tests: Hypothyroidism predisposes to statin-induced rhabdomyolysis 1
- Genetic testing: For recurrent episodes or family history of neuromuscular disorders, consider RYR1, CACNA1S, CPT2, PYGM, ACADM, AMPD1, and VLCAD gene testing 1
- Viral studies and autoimmune markers (ANA, ASMA, ANCA) if viral or autoimmune myositis is suspected 1