Cardiac Enzymes and Markers for Myocardial Infarction

Cardiac biomarkers should be measured in all patients who present with chest discomfort consistent with acute coronary syndrome (ACS).

• Cardiac Troponins T and I are the preferred markers for myocardial injury because they are more sensitive and more specific than the creatine kinase MB isoenzyme (CK-MB), aspartate aminotransferase (AST) or lactate dehydrogenase (LAD), which have much lower specificity to cardiac muscle.
• Patients with negative cardiac biomarkers within 6 hours of the onset of symptoms that are consistent with ACS should have biomarkers re-measured in the time frame of 8 to 12 hours after the onset of symptoms.¹
• Elevations of cardiac enzyme levels should be interpreted in the context of clinical and ECG findings.
• Peaks circulating enzyme levels tend to occur earlier and are often higher following successful thrombolytic therapy.

• Any patient presenting with a possible acute coronary syndrome
• Routinely following percutaneous coronary intervention (PCI)
• Routinely following surgical revascularisation (CABG)

• Troponin is a contractile protein that normally is not found in serum. It is released only when myocardial necrosis occurs.²
• Cardiac troponins T and I are highly sensitive and specific for cardiac damage. Troponin I and T are of equal clinical value.³
• Serum levels increase within 3-12 hours from the onset of chest pain, peak at 24-48 hours, and return to baseline over 5-14 days.
• Troponin levels may not be detectable for six hours after the onset of myocardial cell injury. The most sensitive early marker for myocardial infarction is CK-MB followed by myoglobin.
• Troponin levels should be measured at presentation and again twelve hours after the onset of symptoms. When there is uncertainty regarding the time of symptom onset, troponin should be measured at twelve hours after the presentation.³
• The risk of death from an acute coronary syndrome is directly related to troponin level and patients with no detectable troponins have a good short-term prognosis.
• Elevated troponin levels can occur in patients without an acute coronary syndrome and are associated with adverse outcomes in many other clinical situations, including congestive heart failure, sepsis, acute pulmonary embolism and chronic renal failure.³

• Myocardial muscle creatine kinase (CK-MB) is found mainly in the heart.
• CK-MB levels increase within 3-12 hours of onset of chest pain, reach peak values within 24 hours, and return to baseline after 48-72 hours.
• Sensitivity and specificity are not as high as for troponin levels.

• Serum lactate dehydrogenase (LAD) level rises above the reference range within 24 hours of a myocardial infarction, reaches a peak within 3-6 days, and returns to the baseline within 8-12 days.

• Myoglobin is found in cardiac and skeletal muscle.
• It is released more rapidly from infarcted myocardium than troponin and CK-MB and may be detected as early as 2 hours after an acute myocardial infarction.
• Myoglobin has high sensitivity but poor specificity. It may be useful for the early detection of myocardial infarction.

Studies in several types of acute coronary syndromes have shown that elevated levels of natriuretic peptides, e.g. B-type natriuretic peptide (BNP), are independently associated with adverse outcomes, especially mortality.⁴

• Leucocytosis may be seen within several hours after an acute myocardial infarction. It peaks in 2-4 days and returns to normal levels within 1 week.
• Patients without biochemical evidence of myocardial necrosis but with elevated CRP level are at increased risk of a subsequent ischaemic event.
• Erythrocyte sedimentation rate (ESR) rises above reference range values within 3 days and may remain elevated for weeks.