CKMB Interpretation in Context of Elevated Total CK
A CKMB of 6.2 ng/mL is NOT elevated when total CK is 546 U/L, as this represents only approximately 1.1% of total CK, well below the diagnostic threshold for myocardial injury. 1
Understanding the CKMB Relative Index
The critical factor in interpreting CKMB is not the absolute value alone, but the CKMB relative index (CKMB divided by total CK, expressed as a percentage). 1
- Myocardial tissue contains approximately 15-20% CKMB, while skeletal muscle contains only 1-3% CKMB under normal conditions 2
- A CKMB relative index >5% is required to suggest myocardial origin of the enzyme elevation 1
- In your case: (6.2 ÷ 546) × 100 = 1.1%, indicating the CK elevation is from skeletal muscle, not myocardium 2, 3
Clinical Interpretation
This pattern indicates skeletal muscle injury as the source of CK elevation, not cardiac injury. 2, 3
Common causes of isolated total CK elevation with normal CKMB relative index include:
- Intramuscular injections (including medications like Acuphase) 4, 2
- Strenuous exercise or physical exertion 5
- Cardioversion or defibrillation 3
- Seizures or muscle trauma 2
- Neuromuscular disorders 2
Diagnostic Thresholds for Myocardial Injury
For post-procedural myocardial injury (such as after PCI), CKMB must exceed 5 times the upper limit of normal to be considered diagnostic of MI, not just above the reference range. 1
- Even if CKMB were mildly elevated in absolute terms, without a CKMB relative index >5%, myocardial injury is unlikely 2, 3
- Troponin is the preferred biomarker for cardiac injury due to superior cardiac specificity compared to CKMB 6, 4
Critical Pitfalls to Avoid
Do not misinterpret any CKMB elevation as cardiac injury without calculating the relative index. 2
- Skeletal muscle can release small amounts of CKMB, particularly with trauma, injections, or regenerative muscle conditions 4, 2
- Always correlate with clinical context: presence of chest pain, ECG changes, and troponin levels 4
- In marathon runners, CKMB can reach 26 times the upper limit of normal from skeletal muscle alone, with normal cardiac imaging 5
Recommended Approach
If cardiac injury is suspected despite the low CKMB relative index, measure troponin instead. 6, 4
- Troponin has superior cardiac specificity and is not confounded by skeletal muscle injury 6
- Serial troponin measurements at 0 and 6-9 hours are recommended if acute coronary syndrome remains in the differential 7
- Look for additional evidence of cardiac injury: ischemic symptoms, ECG changes, or imaging abnormalities 4