Cardiac enzymes are substances released by the heart muscle when it is injured - for example, during a heart attack or a severe case of angina.
A heart attack (myocardial infarction) can present in many ways, the most common of which is chest pain. Taking blood tests for heart enzymes can help to make the diagnosis of a heart attack.
When are cardiac enzymes measured?
Patients presenting with chest pain or shortness of breath to healthcare professionals is very common. There can be many causes of these symptoms. Cardiac enzymes are substances released by the heart muscle when it is injured - for example, during a heart attack (myocardial infarction) or a severe case of angina. There are a number of cardiac enzymes and the most common of these is called troponin. There are two types of troponin that can be measured from a blood sample: T and I. Most hospitals will usually only measure one or the other, and both are equally sensitive. Often patients may require a repeat blood test several hours after the first, especially if the first one is negative.
Before troponins, different blood tests were checked looking for heart muscle damage. This included creatine kinase-MB (CK-MB); however, such tests have been outdated by troponins.
What is troponin?
Troponin is a protein released from the heart cells when they are damaged. It is only found in the heart muscle, making it useful in diagnosing damage to the heart muscle. It is important that the blood test result be viewed in conjunction with what the patient has presented with and the heart tracing (12-lead electrocardiogram, or ECG). Taken together these three factors will help make an accurate diagnosis.
Once heart muscle damage occurs, it can take 3-12 hours for the troponin level to increase in the blood. It will usually peak at around 24-48 hours and then gradually return to normal over 5-14 days. Many hospitals will measure troponin after 4-6 hours of the onset of symptoms, and some will run a repeat test after 12 hours. The level of the troponin is directly related to how much heart muscle has been damaged. This means that the higher the level of the troponin, the greater the level of heart muscle damage. The higher the level in a heart attack, the greater the risk of a worse outcome.
Further reading and references
Acute coronary syndrome; Scottish Intercollegiate Guidelines Network - SIGN (2016)
2014 ESC/EACTS Guidelines on myocardial revascularization; The Task Force on Myocardial Revascularization of the European Society of Cardiology and the European Association for Cardio-Thoracic Surgery (Aug 2014)
Myocardial infarction with ST-segment elevation: The acute management of myocardial infarction with ST-segment elevation; NICE Clinical Guideline (July 2013)
Myocardial infarction: cardiac rehabilitation and prevention of further MI; NICE Clinical Guideline (November 2013)
Mehta LS, Beckie TM, DeVon HA, et al; Acute Myocardial Infarction in Women: A Scientific Statement From the American Heart Association. Circulation. 2016 Mar 1133(9):916-47. doi: 10.1161/CIR.0000000000000351. Epub 2016 Jan 25.
2015 ESC Guidelines for the management of acute coronary syndromes in patients presenting without persistent ST-segment elevation; European Society of Cardiology (August 2015)
Unstable angina and NSTEMI; NICE Clinical Guideline (March 2010 - last updated November 2013)
Assessing fitness to drive: guide for medical professionals; Driver and Vehicle Licensing Agency
Ibanez B, James S, Agewall S, et al; 2017 ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation: The Task Force for the management of acute myocardial infarction in patients presenting with ST-segment elevation of the European Society of Cardiology (ESC). Eur Heart J. 2017 Aug 26. doi: 10.1093/eurheartj/ehx393.
Valgimigli M, Bueno H, Byrne RA, et al; 2017 ESC focused update on dual antiplatelet therapy in coronary artery disease developed in collaboration with EACTS: The Task Force for dual antiplatelet therapy in coronary artery disease of the European Society of Cardiology (ESC) and of the European Association for Cardio-Thoracic Surgery (EACTS). Eur Heart J. 2017 Aug 26. doi: 10.1093/eurheartj/ehx419.