Cardiac Enzyme Studies
Cardiac enzyme studies measure the levels of the enzyme creatine phosphokinase (CPK, CK) and the protein troponin (TnI, TnT) in the blood. 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 health professional 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.
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.
Test results are usually available within an hour.
Values and units for reporting the results of cardiac enzyme tests vary considerably from lab to lab. The values listed below can be used as a general guide. Check with your lab or doctor for specific values.
TnI: Less than 0.3 micrograms per liter (mcg/L)
TnT: Less than 0.2 mcg/L
Elevated troponin may be present when you have heart muscle injury. Blood levels of troponin typically rise within 4 to 6 hours after a heart attack, reach their highest levels within 10 to 24 hours, and fall to normal levels within 10 days.
CPK levels generally rise within 4 to 8 hours after a heart attack, reach their highest levels within 12 to 24 hours, then return to normal within 3 to 4 days.
0% to 6% of total CPK (0.00 to 0.06)
CPK-MB is found in large amounts in the heart muscle. A CPK-MB that is higher than normal may be present when you have muscle damage caused by a heart attack. Blood levels of CPK-MB typically rise within 2 to 6 hours after a heart attack, reach their highest levels within 12 to 24 hours, and fall to normal levels within 3 days.
An ongoing high level of CPK-MB levels after 3 days may mean that a heart attack is progressing and more heart muscle is being damaged.
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
- Cardiac enzyme and protein levels must always be compared with your symptoms, past health, physical examination, and electrocardiography (EKG, ECG) results.
- Troponin is an accurate method for quickly diagnosing heart attack, but because it takes up to 6 hours for the troponin level to rise, it can be low or negative at first. Troponin is more specific to heart muscle and remains in the bloodstream longer than CPK.
- 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.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
|Author||Robin Parks, MS|
|Associate Editor||Tracy Landauer|
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||Stephen Fort, MD, MRCP, FRCPC - Interventional Cardiology|
|Last Updated||August 21, 2009|
Last Updated: August 21, 2009