Cardiac markers are substances that are released into the blood when the heart is stressed or damaged. These are often referred to as biomarkers and are helpful in evaluating heart function. The measurements of these biomarkers are vital in the diagnosis of cardiac ischemia and acute coronary syndrome (ACS), these are conditions that are associated with insufficient blood flow to the heart.
Conditions when the heart is under severe stress causes cardiac markers to show up in the blood, for example after a heart attack. In these situations the levels of biomarkers present can be quickly used to find out how seriously the heart was affected and the size of the heart attack.
The cardiac markers listed below are often routinely used in diagnosing a heart attack.
- Cardiac Troponin (TnT and cTn-I): Are the most common biomarkers used, offering the highest sensitivity and are now widely accepted as the best markers for finding a heart attack. Can enter into the bloodstream straight after a heart attack and can stay in the bloodstream for many days, even after all the other biomarkers have returned back to normal levels. Cardiac troponins are specific for heart muscle and several studies have even found a direct correlation between cardiac troponin levels and long term outcome following a chest discomfort episode. This suggest that it is an instrumental tool in helping to identify individual’s that may be either high or low risk for future heart problems. For troponin T, the expected normal reference value is less than 0.1ng/mL and for troponin I, then it is less than 1.5 ng/mL.
- Creatinine Kinase (CK): The levels of this enzyme can usually double after a heart attack. However, it is not very specific since many other conditions are also known to elevate the levels of CK besides a heart attack. The main function of creatinine kinase is to transfer a phosphate group from ATP to creatine. It is essentially made up of B and/or M subunits (CK-BB, CK-MM and CK-MB isoenzymes). The normal reference value is between 30-180 units/L for males and 90-150 for females.
- Creatinine Kinase-MB (CK-MB): A subtype of CK and is more sensitive for measuring heart damage following a heart attack. Levels of CK-MB are found to rise 3-12 hours after a heart attack and generally return to normal levels are 1-2 days. CK-MB is regarded as the benchmark for cardiac markers of myocardial injury and it is also used to demonstrate if thrombolytic therapy has been successful. This is confirmed by the unmodified MB form being rapidly flushed into the blood and become the dominant form. The normal reference value ranges between 10-20 units/L.
- Myoglobin: A small protein that functions in the storing of oxygen, this is not specific in finding a heart attack but is often used alongside troponin to help diagnose a heart attack. Myoglobin can be found both in myocardial and skeletal muscle, it is rapidly released following tissue injury or trauma. The levels can be elevated within an hour of the injury. The normal reference values are below 110 ng/mL.
Other proteins which are also useful cardiac markers include C-reactive protein (CRP), ferritin, haptoglobin, cystatin, fibrinogen, homocysteine and natriuretic peptides especially B-type natriuretic peptide (BNP). It is important that cardiac biomarkers are measured in all patients who experience chest discomfort consistent with acute coronary syndrome (ACS). Any presence of elevated levels cardiac enzymes should be interpreted in the context of ECG and alongside clinical findings.