There are several blood tests that can be used to help diagnose cardiac damage. Some of the most common cardiac marker blood tests include troponin tests, creatine kinase-MB (CK-MB) tests, myoglobin tests, and lactate dehydrogenase (LDH) tests. Each of these markers is released into the bloodstream when the heart muscle is damaged, such as during a heart attack. However, some markers are more specific to the heart than others and can more definitively indicate if cardiac damage or injury has occurred.
Troponin
Of all the cardiac markers, troponin levels are considered the most sensitive and specific indicator of cardiac damage. Troponins are regulatory proteins found inside heart muscle cells that help regulate muscular contraction. There are three types of troponins: troponin T, troponin I, and troponin C. When the heart muscle is injured, troponins T and I are released into the bloodstream.
Since troponins are found almost exclusively in heart muscle cells, an elevated troponin level strongly suggests that damage to the heart has occurred. Even minor elevations in troponin above the normal range may indicate minor cardiac injury or stress. More pronounced troponin elevations usually correlate with more significant damage to the heart muscle.
Troponin levels begin to rise 2-4 hours after cardiac injury and may remain elevated for up to 2 weeks, making them useful for both early diagnosis and monitoring of heart damage over time. Troponin is such a sensitive marker that even small amounts of cardiac injury undetectable by other tests may result in detectable troponin elevations. For this reason, troponin is considered the “gold standard” cardiac marker test.
Creatine Kinase-MB
Creatine kinase (CK) is an enzyme found in heart muscle cells and other tissues in the body. When the heart muscle is damaged, CK isoenzymes will leak into the bloodstream. CK has three isoenzymes: CK-MM, CK-BB, and CK-MB. The CK-MM isoenzyme is found primarily in skeletal muscle, CK-BB is found mainly in brain tissue, and CK-MB is found predominantly in cardiac muscle cells.
An elevated CK-MB level indicates damage to heart muscle and is used as a diagnostic marker for myocardial infarction (heart attack). CK-MB begins to rise about 4-6 hours after symptom onset and returns to normal within 48-72 hours. However, since CK-MB is also found in small amounts in skeletal muscle, an elevated level may occasionally be caused by skeletal muscle injury rather than cardiac damage. For this reason, CK-MB is not quite as specific for cardiac injury as troponin. Nonetheless, CK-MB is still used as an important marker for diagnosis of acute myocardial infarction, especially when trended over time.
Myoglobin
Myoglobin is an oxygen-binding protein found in cardiac and skeletal muscle tissue. It is released rapidly into the bloodstream when muscle cells are damaged. Elevated myoglobin can indicate a heart attack or skeletal muscle injury. However, myoglobin lacks cardiac specificity since it rises with any striated muscle damage.
Myoglobin levels begin to rise within 2-4 hours of muscle injury and return to normal within 24 hours. The main usefulness of the myoglobin blood test is for early detection of a heart attack. An elevated myoglobin level may be detected earlier than troponin or CK-MB since it appears in the blood sooner after heart muscle injury occurs. However, the test is less specific for detecting cardiac damage. For this reason, myoglobin is most useful when trended serially along with troponin and CK-MB testing.
Lactate Dehydrogenase
Lactate dehydrogenase (LDH) is an enzyme found in many tissues throughout the body. LDH catalyzes the conversion of lactate to pyruvate. When cells are damaged, LDH leaks out into the bloodstream. Therefore, an elevated LDH level indicates general cellular damage or injury, which is not specific to the heart.
In the setting of acute myocardial infarction, LDH levels begin to rise 24-48 hours after the onset of symptoms and remain elevated for up to 10 days. However, since many organs contain LDH, an elevated level does not necessarily indicate the heart as the source. LDH can also be elevated due to liver disease, kidney disease, skeletal muscle breakdown, hemolytic anemia, and cancer. For this reason, LDH is the least specific cardiac marker. However, trends in LDH values over several days may still provide useful supportive information when interpreted in the proper clinical context along with other marker tests.
Summary
– Troponin (troponin I or troponin T) is the most cardiac-specific marker and even minor elevations are indicative of myocardial injury. Troponin levels begin to rise 2-4 hours after injury and remain elevated for up to 2 weeks.
– CK-MB is fairly specific for cardiac damage, especially when trended over time, but may also be elevated with skeletal muscle injury. CK-MB levels rise 4-6 hours after injury and return to normal within 48-72 hours.
– Myoglobin rises rapidly within 2-4 hours of cardiac injury but is less cardiac-specific since it is also found in skeletal muscle. It returns to normal within 24 hours.
– LDH is the least specific marker since it indicates general cellular injury rather than specific cardiac damage. LDH begins to rise 24-48 hours after symptom onset.
In summary, cardiac troponins are considered the most sensitive and specific blood tests for detecting cardiac injury. Even minor troponin elevations provide evidence of myocardial damage. Troponins are the preferred diagnostic and prognostic cardiac markers measured in patients with suspected acute coronary syndrome or myocardial infarction. However, looking at trends of multiple markers over the early hours and days of symptoms onset provides the most comprehensive information.
Frequently Asked Questions
Why are troponins so specific for cardiac injury?
Troponins are specific for cardiac damage because they are proteins found almost exclusively in heart muscle tissue. Troponin T and I isoforms in particular represent about 5-6% of the total protein in cardiac myocytes but are not found in significant amounts in skeletal muscle or other cell types. When the membrane around heart muscle cells is injured, as occurs in a heart attack, troponins leak out into the blood.
Which troponin test is better – troponin I or troponin T?
Both troponin I and T are highly specific markers of cardiac injury. Troponin I may rise slightly sooner, while troponin T may remain elevated slightly longer. However, both are sensitive markers of even minor myocardial damage. Most laboratories measure troponin T rather than I, but both are considered equivalent in diagnosing cardiac injury.
Why are troponins superior to CK-MB for detecting a heart attack?
Troponins are more cardiac-specific than CK-MB. CK-MB can also be elevated due to skeletal muscle damage, which can occasionally confound the diagnosis. Furthermore, troponin levels rise earlier than CK-MB after a heart attack, allowing more rapid diagnosis. Troponins also remain elevated longer, which aids in monitoring for recurrent cardiac injury. Even small amounts of myocardial damage can cause detectable troponin elevations, making them extremely sensitive markers.
Can renal failure cause elevations in cardiac markers like troponin and CK-MB?
Yes, renal failure can cause mild elevations in troponin and CK-MB, even without actual cardiac injury. This occurs because the damaged kidneys have a harder time clearing these enzymes from the bloodstream. However, the elevations due to renal failure alone are usually minor compared to the large elevations seen with substantial myocardial damage. Looking at the overall clinical picture helps distinguish renal failure-related elevations from those due to an acute heart attack or other cardiac damage.
How long after a heart attack do troponin levels remain elevated?
Troponin levels begin to rise within 2-4 hours of cardiac injury, peak at 24-48 hours, and may remain elevated for up to 2 weeks after a heart attack. The precise duration depends on the extent of myocardial damage. With minor injury, troponin may return to normal within a few days. Large heart attacks cause more prolonged troponin elevations that can persist for 7-14 days. Troponin values should gradually trend down over time, and failure to do so may indicate recurrent cardiac injury or an ongoing complication.
Key Takeaways
– Cardiac troponins are the most sensitive and specific blood marker for detecting myocardial injury, even minor damage.
– Troponin elevations indicate high likelihood of underlying cardiac pathology.
– CK-MB is also used but is less specific than troponins since it can also rise due to skeletal muscle injury.
– Myoglobin is an early marker that rises rapidly but lacks cardiac specificity.
– LDH is the least helpful since it indicates general cellular damage rather than specifically cardiac injury.
– Looking at trends of multiple cardiac markers over time provides the best diagnostic assessment when evaluating possible acute coronary syndrome.
Conclusion
Cardiac marker blood tests play an important role in diagnosing and evaluating the severity of heart damage, especially during a myocardial infarction. While several markers can become elevated, cardiac troponins I and T are considered the gold standard laboratory tests. Even minor troponin elevations are highly indicative of underlying cardiac injury. When interpreted in the proper clinical context, troponin levels can help identify heart attacks rapidly, guide treatment decisions, and provide prognostic information. Troponins remain the most sensitive and specific blood test for detecting cardiac damage.
