For a patient suspected of having rhabdomyolysis, do you check Creatine Kinase (CK) or Creatine Kinase-MB (CK-MB)?

Rhabdomyolysis: Check Total CK, Not CK-MB

For suspected rhabdomyolysis, you should check total creatine kinase (CK), not CK-MB. CK-MB is a cardiac-specific isoenzyme used for myocardial infarction diagnosis and lacks the sensitivity and specificity needed for rhabdomyolysis assessment 1, 2.

Why Total CK is the Correct Test

Total CK is the diagnostic standard for rhabdomyolysis, with a reasonable diagnostic threshold being CK elevation of at least 10 times the upper limit of normal, followed by a rapid decrease toward normal values 3. In clinical practice, CK levels can reach 100,000 IU/L or higher in severe cases 3.

• CK-MB is specifically elevated after cocaine use due to skeletal muscle injury and can be falsely positive in the absence of true myocardial infarction, which is why troponin I and troponin T are preferred for cardiac injury assessment in this context 1
• The guideline explicitly states that "increased motor activity, skeletal muscle injury, and rhabdomyolysis can occur, causing CK and even CK-MB elevation in the absence of MI" 1

Diagnostic Approach and Monitoring

Order total CK along with comprehensive metabolic panel including creatinine, electrolytes (particularly potassium), calcium, phosphorus, and urinalysis for myoglobin 4, 2.

Initial Laboratory Workup

• Complete electrolyte panel is crucial, as hyperkalemia can lead to life-threatening cardiac arrhythmias 2
• Urinalysis showing brown color, cloudiness, and positive for blood without RBCs indicates myoglobinuria 2
• Liver function tests (AST, ALT) should be obtained, as transaminases are commonly elevated due to muscle enzyme release 2
• Additional muscle enzymes including LDH, aldolase may be measured as they are commonly elevated 2

Timing Considerations

CK levels peak between 24-120 hours after the inciting event, not immediately 2. At 9 hours post-injury, CK levels are typically still rising and have not reached their maximum 2.

• If clinical suspicion remains high but initial CK is normal or only mildly elevated, repeat CK measurement at 24 hours post-event 2
• The large molecular size of CK (82 kDa) requires lymphatic clearance before entering the bloodstream, explaining the delayed peak 2

Risk Stratification Based on CK Levels

Acute kidney injury develops in over 80% of crush syndrome patients when CK exceeds 75,000 IU/L 4. However, AKI risk exists at lower thresholds:

• CK >16,000 IU/L is associated with increased risk of acute tubular necrosis 3
• Peak CK ≥5,000 IU/L has 83% sensitivity for predicting AKI requiring renal replacement therapy 5
• CK levels of 3,000-5,000 IU/L are considered abnormal and possibly associated with increased AKI risk in clinical populations 2

Serial Monitoring Strategy

Trend CK, creatinine, and electrolytes daily until CK is declining and renal function is stable 2.

• Monitor potassium closely and correct hyperkalemia emergently 2
• Maintain hourly urine output monitoring and target urine pH >6.5 in established cases 4
• Myoglobin measurement may be more sensitive than CK for early identification of AKI risk, as myoglobin peaks earlier in plasma 4

Common Pitfalls to Avoid

Do not rely on CK-MB for rhabdomyolysis diagnosis—it is neither sensitive nor specific for skeletal muscle injury and will lead to diagnostic confusion 1.

• Impact trauma from falls can drastically increase CK without reflecting true muscle breakdown, but given clinical context, assume true rhabdomyolysis until proven otherwise 2
• In acute kidney injury settings, creatinine interpretation becomes complex because creatinine production falls due to reduced hepatic creatine synthesis while muscle injury increases creatinine release 4
• CK is not a specific or early predictor of AKI—a McMahon score ≥6 calculated on admission allows for more sensitive, specific, and timely identification of high-risk patients (68% specific, 86% sensitive for RRT) 5