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Cardiac enzymes

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 medical professionals to diagnose a heart attack.

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What are cardiac enzymes?

When your heart muscle is damaged, cardiac enzymes are released into your bloodstream. Enzymes are proteins that speed up chemical reactions and biological processes in your body. Cardiac enzymes are also sometimes called cardiac biomarkers. The cardiac enzyme levels that are tested are called cardiac troponins. Normal (skeletal) muscle also produces troponins but troponins T and I are more specific for heart muscle and so are the cardiac enzymes that are usually tested.

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 This included creatine kinase-MB (CK-MB); however, such tests have been outdated by troponins.

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 released by the heart muscle when it is injured - for example, during a heart attack (myocardial infarction) or a severe case of angina. Therefore cardiac enzyme levels can be used to test for a heart problem such as a heart attack.

Of the 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.

How long does a cardiac enzyme test take?

Testing for cardiac enzymes just needs a simple blood sample to be taken and so it is very quick and easy. The results of the test will also be available quickly but will be much quicker if you are in hospital rather than in your General Practice, but even then the result will usually be available by the following day. The test is performed quickly because high levels of cardiac enzymes may indicate a serious diagnosis, such as a heart attack, that will need to be treated very urgently.

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Is troponin a cardiac enzyme?

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 levels 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

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The information on this page is written and peer reviewed by qualified clinicians.

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