Cardiac Biomarkers and Cardiac Enzymes

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When heart muscle cells (myocardial cells) are injured because of myocardial infarction (heart attack) or trauma, certain proteins that reside inside these cells - in particular, creatine kinase and troponin - leak out into the bloodstream. Measuring blood levels of these proteins can help doctors determine whether cardiac damage has occurred.

”Cardiac Enzyme Test” vs. “Biomarker Test”

The blood test that looks for these telltale proteins in the blood used to be called the “cardiac enzyme test” (because creatine kinase, an enzyme, was once the only measurement made).

However, troponin is now the more important measurement, and troponin is not an enzyme. (It is a complex of regulatory proteins important to the contraction of cardiac muscle.) So the blood tests that look for heart muscle damage are now referred to as biomarker tests.

How are Biomarker Tests Used?

Measuring biomarkers is usually an important early step in diagnosing a myocardial infarction.

Today, troponin is the preferred biomarker for this purpose. Troponin is more specific marker for heart muscle damage than creatine kinase and is also a more sensitive marker. Most doctors will still measure both troponin and creatine kinase levels when a heart attack is suspected - but whether the creatine kinase measurement adds much to clinical care is questionable.

Confirming that a heart attack has occurred generally requires several blood tests, demonstrating a rise and fall of blood biomarkers over time.

Creatine kinase is released into the bloodstream 4 to 6 hours after heart cell damage occurs, and peak blood levels of creatine kinase are seen after 24 hours. Elevated creatine kinase levels usually, but not always, indicate heart muscle damage. Creatine kinase levels sometimes can be increased with damage to other kinds of cells as well.

Troponin is released into the bloodstream 2 to 6 hours after heart cell damage, and blood levels peak in 12 to 26 hours. Elevated levels of troponin are regarded as a more reliable indicator of heart muscle damage than elevated creatine kinase levels.

Because troponin is an "earlier" marker of cardiac cell damage than creatine kinase, and because it is somewhat more accurate at indicating heart cell damage than creatine kinase, troponin is the preferred marker today for diagnosing myocardial infarction.

When Are Biomarkers Most Helpful?

When a patient has a typical myocardial infarction with ST-segment elevation on the ECG (a “STEMI”), it is generally not necessary for the doctor to wait for the results of the biomarker test before initiating treatment.

Biomarkers are more helpful in heart attack patients who do not have typical ECG changes (an “NSTEMI”). In these cases, especially if the initial biomarker blood test is in the “indeterminate” range, a second blood test a few hours later that shows a rising troponin (or CK) level will reveal the diagnosis.

In recent years, a high-sensitivity troponin assay has been developed that, in some patients, allows the diagnosis of a heart attack with a single blood test - and thus, allows treatment to begin earlier than otherwise might be advisable.

Source:

Thygesen, K, Alpert, JS, White, HD, et al. Universal definition of myocardial infarction: Kristian Thygesen, Joseph S. Alpert and Harvey D. White on behalf of the Joint ESC/ACCF/AHA/WHF Task Force for the Redefinition of Myocardial Infarction. Eur Heart J 2007; 28:2525.

Mills NL, Churchhouse AM, Lee KK, et al. Implementation of a sensitive troponin I assay and risk of recurrent myocardial infarction and death in patients with suspected acute coronary syndrome. JAMA 2011; 305:1210.

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