Cardiac biomarkers in acute coronary syndrome

What it is

Acute coronary syndrome is the clinical manifestation of myocardial infarct, and commonly the working diagnosis in a patient with new-onset chest pain suspected to be of cardiac ischemic origin. Clinical findings in combination with ECG and troponin are the mainstays of diagnosis, so the biomarker is one part of the answer and never the whole of it.

The four boxes are set by ST elevation together with the biomarkers:

• Unstable angina: no ST elevation, biomarkers negative, symptoms at rest.
• Stable angina: no ST elevation, biomarkers negative, symptoms on exertion under 15 min.
• NSTEMI: no ST elevation, biomarkers positive.
• STEMI: ST elevation, biomarkers positive.

How it is diagnosed

In myocardial infarction the laboratory finding is a rising troponin T/I, in contrast to angina pectoris. CK and CK-MB give the extent of cell damage, and BNP and myoglobin are also part of the panel. In stable angina, by contrast, cardiac enzymes (CK-MB/troponin) are normal.

When each one rises, peaks and returns is what makes the choice between them:

• Troponin T/I: rises at 6-8 hours with regular assays and at 1-3 hours with high-sensitive assays, maximum at 12-24 hours, normalization over 7-10 days.
• CK-MB: rises at about 4-9 hours, maximum at 12-24 hours, normalization over 2-3 days.
• Myoglobin: rises at about 1 hour, maximum at 4-12 hours, normalization by 24 hours.

Myoglobin therefore rises first and returns to normal first, while troponin stays elevated for days, which is why troponin still finds an infarct in a patient who presents late.