Test Overview

Cardiac enzyme studies measure the levels of enzymes and proteins that are linked with injury of the heart muscle. These include the enzymes creatine phosphokinase (CPK) and creatine kinase (CK), and the proteins troponin I (TnI) and troponin T (TnT). Low levels of these enzymes and proteins are normally found in your blood, but if your heart muscle is injured, such as from a heart attack, the enzymes and proteins leak out of damaged heart muscle cells, and their levels in the bloodstream rise.

Because some of these enzymes and proteins are also found in other body tissues, their levels in the blood may rise when those other tissues are damaged. Cardiac enzyme studies must always be compared with your symptoms, your physical examination findings, and electrocardiogram (EKG, ECG) results.

Why It Is Done

Cardiac enzyme studies are done to:

• Determine whether you are having a heart attack or a threatened heart attack (unstable angina) if you have chest pain, shortness of breath, nausea, sweating, and abnormal electrocardiography results.
• Check for injury to the heart after bypass surgery.
• Determine if a procedure, such as percutaneous coronary intervention (PCI), or a medicine to dissolve the blockage (thrombolytic medicine) has successfully restored blood flow through a blocked coronary artery.

How To Prepare

No special preparation is required before having this test.

Many medicines may affect the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

Cardiac enzyme studies are often repeated over several hours for comparison. Blood samples for these cardiac enzyme tests are usually drawn every 8 to 12 hours for 1 to 2 days after a suspected heart attack, to look for the rise and fall in the enzyme levels.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin) and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

Cardiac enzyme studies measure the levels of the enzymes creatine phosphokinase (CPK) and creatine kinase (CK), and the proteins troponin I (TnI) and troponin T (TnT) in the blood.

Values and units for reporting the results of cardiac enzyme tests vary considerably. The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Troponin normal values:¹

• TnI: Less than 0.35 micrograms per liter (mcg/L)
• TnT: Less than 0.2 mcg/L

Total CPK (creatine phosphokinase) normal values:¹

• 0–120 micrograms per liter (mcg/L)

CK-MB (creatine kinase) normal values:¹

• 0–3 micrograms per liter (mcg/L)

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Other diseases, such as muscular dystrophy, certain autoimmune diseases, and Reye syndrome.
• Other heart conditions, such as myocarditis and some forms of cardiomyopathy.
• Emergency measures to treat heart problems, such as CPR, cardioversion, or defibrillation.
• Medicines, especially injections into muscles (IM injections).
• Cholesterol-lowering medicines (statins).
• Heavy alcohol use.
• Recent strenuous exercise.
• Kidney failure.
• Recent surgery or serious injury.

What To Think About

• CPK-MB, which is found in large amounts in damaged heart muscle is a more specific way to estimate the amount of heart muscle damage than total CPK. The total CPK enzyme level can be elevated from vigorous exercise, intramuscular injections, crush injuries to muscles, muscular dystrophy, or muscle inflammation.
• Another protein, myoglobin, may be tested along with cardiac enzymes to diagnose a heart attack.

Related Information

• Heart Attack and Unstable Angina

References

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