What is being tested?
Creatine kinase-MB (CK-MB) is a form of an enzyme found primarily in heart muscle cells. This test measures CK-MB in the blood.
CK-MB is one of three forms (isoenzymes) of the enzyme creatine kinase (CK). These isoenzymes include:
- CK-MM (found in skeletal muscles and the heart)
- CK-MB (found mostly in the heart, but small amounts found in skeletal muscles)
- CK-BB (found mostly in the brain and smooth muscle, such as the intestines and uterus)
CK is released from muscle cells and is detectable in the blood whenever there is muscle damage. The small amount of CK that is normally in the blood is primarily CK-MM. CK-BB almost never gets into the blood, and CK-MB will typically only be present in significant amounts when the heart is damaged. A CK test measures the total level but does not distinguish between the three isoenzymes. When there is an increased amount of CK present in the blood, the CK-MB test can be used to determine whether it is due to heart damage or is more likely to be related to skeletal muscle injury.
How is it used?
A creatine kinase-MB (CK-MB) test may be used as a follow-up test to an elevated creatine kinase (CK) in order to determine whether the increase is due to heart damage or skeletal muscle damage. The test is most likely to be ordered if a person has chest pain or if a person’s diagnosis is unclear, such as if a person has nonspecific symptoms like shortness of breath, extreme fatigue, dizziness, or nausea.
CK and CK-MB were once the primary tests ordered to detect and monitor heart attacks, but they have now been largely replaced by the troponin test, which is more specific for damage to the heart.
Sometimes, the CK test may be used if a heart attack is suspected and a troponin test is not available. In this case, when CK is elevated, a CK-MB test may be used as a follow-up test to determine whether the increase is due to heart damage or skeletal muscle damage.
When is it ordered?
CK-MB is usually ordered along with or following an elevated total CK when a person has chest pain and a healthcare practitioner wants to determine whether the pain is due to a heart attack. It is typically ordered for this purpose when the more specific troponin test is not available. A CK-MB may also be ordered when a person has a high CK to determine whether the muscle damage detected is to the heart or other muscles.
Increased CK-MB can usually be detected in someone with a heart attack about 3-6 hours after the onset of chest pain. The level of CK-MB peaks in 12-24 hours and then returns to normal within about 48-72 hours. If there is a second heart attack or ongoing damage, then levels may rise again and/or stay elevated longer.
What does the test result mean?
CK-MB is normally undetectable or very low in the blood.
Chest pain and increased CK levels plus elevated CK-MB indicate that it is likely that a person has recently had a heart attack. Levels that drop, then rise again may indicate a second heart attack and/or ongoing heart damage.
If CK-MB is elevated and the ratio of CK-MB to total CK (relative index) is more than 2.5-3, then it is likely that the heart was damaged. A high CK with a relative index below this value suggests that skeletal muscles were damaged. (For more, see Common Questions #3.)
Any kind of heart muscle damage can cause an increase in CK and CK-MB, including physical damage from trauma, surgery, inflammation, and decreased oxygen (ischemia). Strenuous exercise may also increase both CK and CK-MB, but usually with a lower relative index.
Kidney failure can cause a high CK-MB level.
Is there anything else I should know?
Since CK-MB is also present in small quantities in skeletal muscle, significant damage to skeletal muscles can also increase the CK-MB level. If both skeletal and heart muscles are damaged, the presence of CK-MB due to a heart attack could be masked.
If I have chest pain, does that mean I am having a heart attack?
Many other problems can cause chest pain, and it is not always possible to tell just from the type of chest pain whether or not you are having a heart attack. Many people have chest pain from straining the muscles in their chest, and chest pain can occur with some lung problems. Chest pain can be a warning sign of hardening of the arteries of the heart (coronary artery disease or CAD). Chest pain that occurs during exercise, hard work, or at times of stress, lasts for a few minutes and goes away with rest is called angina. If the pain lasts longer than just a few minutes, especially if it occurs when you are resting, seek immediate medical attention.
What other tests detect a heart attack?
Health care practitioners may use more than one test to determine if a person who has chest pain is having a heart attack. Troponin is generally considered the most accurate test. Besides CK-MB, a CK test and a myoglobin test may be performed, although they are less specific as other conditions can also produce an increase in these two tests.
What is CK index?
People with skeletal muscle damage may have elevated CK-MB levels. To help differentiate heart attack from skeletal muscle damage, a CK index can be calculated using CK-MB and total CK as follows: CK index = (CK-MB, ng/mL) x100 / (total CK activity, IU/L). If CK-MB is elevated and the CK index in higher than 2.5 to 3.0, heart damage is likely.