Is creatine phosphokinase (CPK) useful for diagnosing myocardial infarction?

Cardiac Troponins Are Superior to CPK for Diagnosing Myocardial Infarction

Cardiac troponins (cTnT or cTnI) are the preferred biomarkers for diagnosing myocardial infarction, while creatine phosphokinase (CPK) and its MB isoenzyme (CPK-MB) are no longer recommended as primary diagnostic tests. 1

Comparison of Cardiac Biomarkers

Cardiac Troponins

• Superior sensitivity and specificity for myocardial injury compared to CPK-MB 2
• Remain elevated for 7-14 days after infarction, providing a wider diagnostic window 2, 1
• Detect very small amounts of myocardial necrosis that CPK-MB might miss 2
• Powerful tool for risk stratification and therapy selection 2
• Defined as elevated when exceeding the 99th percentile of a normal reference population 2

CPK-MB

• Less sensitive and less specific than troponins for myocardial injury 2
• Shorter elevation window (24-36 hours) compared to troponins 1
• Can be falsely elevated in skeletal muscle injury or disease 2
• Not recommended for primary diagnosis of MI 2
• If used, should be measured by mass immunoassays, not older methods 2

Limited Clinical Scenarios Where CPK-MB May Be Useful

1. Detection of reinfarction: Due to its shorter half-life, CPK-MB may help detect a new MI in patients who recently had one, when troponin levels are still elevated from the first event 2, 1

2. Periprocedural myocardial injury: CPK-MB has established diagnostic value for detecting myocardial injury during cardiac procedures 1

3. When troponin testing is unavailable: In settings where troponin assays are not available, CPK-MB is the best alternative 2

Diagnostic Protocol for Suspected MI

1. Initial biomarker testing:

   • Obtain cardiac troponin levels at presentation 1
   • Repeat measurement 6-9 hours later 2
   • Consider additional sample between 12-24 hours if clinical suspicion remains high but earlier measurements were negative 2

2. Interpretation of results:

   • A rise and/or fall pattern exceeding the 99th percentile URL is essential for diagnosis 2
   • One elevated value above the decision level is required to establish MI diagnosis 2

Common Pitfalls and Caveats

• Total CPK measurement is not recommended for MI diagnosis due to its large skeletal muscle distribution and lack of specificity 2

• Normal total CPK does not exclude MI: Studies have shown MI can occur with normal total CPK values 3, 4

• Post-cardiac arrest interpretation: Cardiac biomarkers (including troponin and CPK-MB) show poor diagnostic performance after successful resuscitation, with mediocre sensitivity and specificity 5

• Non-ischemic causes of troponin elevation: Many conditions can cause troponin elevation without MI, including renal failure, sepsis, pulmonary embolism, and extreme exertion 2, 1

• Timing matters: Sampling too early (<6 hours from symptom onset) may yield false negatives for both troponin and CPK-MB 6, 4

In conclusion, while CPK-MB was historically important for diagnosing MI, cardiac troponins have definitively replaced it as the biomarker of choice due to superior sensitivity, specificity, and a wider diagnostic window. CPK-MB should only be considered in specific scenarios such as diagnosing reinfarction or when troponin testing is unavailable.