Cardiac Enzymes: What They Are and What Test Results Mean
This article explains cardiac enzymes, including why doctors measure them and what the results mean.
Cardiac enzymes refer to protein components inside heart muscle cells. Measuring them in the blood is an important tool doctors have for diagnosing the cause of chest pain. Their presence in the blood indicates that the heart may have sustained damage.
Main cardiac enzymes
The three main cardiac enzymes that doctors can test for are:
- Troponin: Troponin is the main cardiac enzyme. It plays a role in heart contractions and is almost exclusive to the heart muscle. In healthy adults, there is no troponin circulating in the blood. Troponin levels rise within 4 hours of a heart attack. They then peak at 24–48 hours, and they remain high for several days afterward. All of this makes troponin a very specific and useful test.
- Creatine kinase (CK) or creatine phosphokinase (CPK): These enzymes are found in the heart, muscles, and brain. In general, they help muscles produce energy. Before troponin enzyme tests, CK or CPK were the mainstays of cardiac markers.
- Myoglobin: This protein is found in the heart and skeletal muscles. It supplies oxygen to cells, helping them produce energy for the muscle to work. Excess levels of myoglobin are released into the bloodstream due to muscle injuries. This can lead to kidney disease. This test is often run along with CK.
This article goes into detail about what these three cardiac enzyme tests may reveal.
Less common cardiac enzymes
Doctors do not routinely check these cardiac enzymes:
- Aspartate aminotransferase (AST): AST is mostly found in the liver and in other muscles and organs, including the heart. This test is more commonly used to check for liver disease, but it is also related to cardiovascular disease. AST was the first biomarker doctors used.
- Copeptin: Copeptin is a peptide. Copeptin levels rise directly after an acute cardiac event, whereas troponin levels continue to rise for 24–48 hours. Doctors may still test for copeptin along with troponin.
- Lactate dehydrogenase (LDH): LDH enzyme helps your body convert glucose to energy for cells to use. Almost all tissues in the body have LDH. When tissues or organs are injured or damaged, the tissues release LDH into the bloodstream. Doctors may measure LDH along with CK.
Another cardiac enzyme, heart-type fatty acid binding protein, is not used in the United States. In some settings, it has shown higher sensitivity to heart damage. However, it is not as specific as troponin. More research is necessary to properly define its role in diagnosing a heart attack.
A cardiac enzyme test measures the amount of heart biomarkers in the blood. Today, the main test is troponin, but doctors may also order CK and myoglobin on occasion. High levels of cardiac enzymes may indicate a heart attack or other heart conditions.
The main use of a cardiac enzyme test is to help diagnose or rule out a heart attack. Doctors may also order cardiac enzyme tests to monitor the heart after cardiac surgery or procedures, such as angioplasty.
Recent research suggests that cardiac enzyme tests may be useful after noncardiac surgery in some cases. Mainly, it may help identify potential heart injury after surgery in people at high risk of cardiovascular disease.
Doctors are most likely to order a cardiac enzyme test if you have symptoms of a heart attack.
Symptoms of a potential heart attack include:
- chest pain, discomfort, pressure, or squeezing
- dizziness, lightheadedness, or fainting
- heart palpitations
- nausea with or without vomiting
- pain that spreads to the arm, shoulder, back, neck, or jaw
- shortness of breath
- sudden anxiety
Seek immediate medical care (call 911) for any of these serious symptoms.
Currently, doctors are most likely to order a troponin test to evaluate the heart. They may also order a CK test or a myoglobin test in some cases.
Lab test reference ranges can vary from one lab to another. Your doctor will share cardiac enzyme test results with you and let you know what they reveal.
There are a few different troponin assays available, and they are not equivalent. This makes it tricky to interpret them and define normal levels. There is also a standard troponin test and a high-sensitivity version.
Normally, troponin is not detectable in the blood of healthy people. A high troponin level is anything above the 99th percentile of the upper reference limit for any particular test. This means the level is higher than 99% of what you would see in a healthy control population.
A standard troponin test measures two troponin components, cardiac troponin I (cTnI) and cardiac troponin T (cTnT). They are measured in nanograms per milliliter (ng/ml). A general reference range for standard troponin tests includes:
- cTnI: 0.4 ng/ml or lower
- cTnT: 0.1 ng/ml or lower
For a high-sensitivity cTnT test, the general cutoff at the upper reference limit is given in nanograms per liter (ng/l) and is sex specific:
- biological females: 10 ng/l or lower
- biological males: 15 ng/l or lower
CK or CPK test
Generally, a normal range for CK is 20–200 international units per liter (IU/l). Anything above this would be high. However, this range may vary by race and sex in healthy people.
CK-MB should be less than 5% of the total CK test value.
A general cutoff for a myoglobin blood test is less than 100 micrograms per liter (mcg/l) or ng/ml.
The meaning of high cardiac enzyme levels depends on which enzyme and what other conditions you have. Your doctor will look at your entire clinical picture when interpreting test results.
The main cause of a high troponin level is a heart attack. However, there are several other cardiac- and noncardiac-related conditions that can increase troponin.
CK-MB can be high after a heart attack. However, it can also be high when there is skeletal muscle damage or after extreme exercise.
Likewise, myoglobin levels can be high after both a heart attack and acute skeletal muscle injury.
Cardiac enzyme tests are blood tests. This usually involves drawing blood from a vein in your arm. Most people will have a catheter placed in an arm vein after admission to the emergency room. If you do not have one, collecting the sample will involve inserting a small needle.
The results typically come back within 60 minutes. Repeat tests are often necessary while diagnosing a heart attack.
Here are some questions people often ask about cardiac enzymes.
What causes elevated cardiac enzymes?
There can be several causes of elevated cardiac enzymes. A heart attack is the major one that doctors try to rule out or confirm using cardiac enzymes. Troponin is the most utilized test for identifying a heart attack. However, other causes can increase troponin and other cardiac enzyme levels.
What can you do for cardiac enzyme levels?
Your doctor will advise you about the next steps in the diagnostic process if cardiac enzyme levels are high. This may involve more testing or procedures, such as angioplasty.
Cardiac enzyme tests are a diagnostic tool doctors use to evaluate chest pain and other symptoms of a heart attack. Troponin is the main test because it is most specific to heart muscle damage. Other conditions can raise troponin and other cardiac enzyme tests. Still, cardiac enzyme tests allow doctors to quickly evaluate the possibility of heart muscle damage.