Overview of Cardiac Enzymes
- Cardiac troponins (T and I) are specific markers of myocardial damage.
- Each medical facility usually utilizes one of these troponins (T or I) in conjunction with CK and CK-MB testing as part of their cardiac enzyme order set.
- It is typical to order a baseline, 6 hr and 12 hr level of a cardiac specific troponin, CK and CK-MB.
Information on Troponin I testing
- The troponin I (cTnI) test measures the concentration of cardiac troponin I subtype in the blood.
- Troponins are a complex group of three contractile proteins (C, T and, I) which regulate the interaction of actin and myosin in cardiac and skeletal muscle.
- Though structurally the cardiac and skeletal subforms of Troponin C are identical, Troponin T and I have distinct cardiac and skeletal subforms which can be identified by specific assay. Hence Troponin I and T are highly cardiospecific.
- Approximately 2% of cTnI and 6% of cTnT are free in the myocardial cytoplasm.
- Myocardial injury or necrosis results in release of cardiac troponins into the bloodstream as early as 2-6 hrs.
- Troponin I is typically positive within 2-6 hours after cardiac injury and returns to baseline within 7-12 days.
- Troponin I shows a biphasic peak with initial peak at 15-24 hours and then a second lower peak at 60-80 hours.
- cTnI can be analyzed by both qualitative (bedside, point of care) and quantitative assays.
The clinical utility of the serum troponin I test includes:
- Detects myocardial injury (hours after it has occurred)
- Used in the evaluation and diagnosis of chest pain
- Aids in the diagnosis of acute myocardial infarction (AMI) & acute coronary syndrome (ACS)
- Is a sensitive marker for unstable angina
- For detection of perfusion following thrombolysis
- As a prognostic marker and for risk stratification in acute coronary syndromes (ACS)
- To monitor for perioperative myocardial infarction
- For diagnosis of cardiac contusion
- For excluding cardiac injury during electroconversion
- As a biomarker in drug induced cardiotoxicity
- Found to be a prognostic indicator of mortality risk in patients with acute decompensated heart failure (a positive result – 8% mortality versus 2.7% mortality in patients with a negative result)
Advantages of cardiac troponin I test include:
- Better specificity and sensitivity for cardiac damage than CK-MB
- Aids in the early detection of small myocardial infarcts that are undetectable by conventional diagnostic methods
- Detects cardiac damage that has occurred up to 7-12 days prior, since it remains elevated for 7-12 days after injury
- cTnI is more sensitive for the diagnosis of acute myocardial infarction than creatinine phosphokinase MB fraction (CK-MB), myoglobin and lactate dehydrogenase (LDH) isoenzymes
Additional information
- Myocardial injury or necrosis results in release of cardiac troponins into the bloodstream as early as 2-6 hrs.
- Troponin I is typically positive within 2-6 hours after cardiac injury and returns to baseline within 7-12 days.
- Troponin I shows a biphasic peak with initial peak at 15-24 hours and then a second lower peak at 60-80 hours.
- Cardiac troponins are ideal biomarkers for evaluation of ACS in patients with coexistent skeletal muscle injury
- Cardiac troponins have the disadvantage of not being as useful for monitoring for reinfarction due to their prolonged elevation following myocardial injury (one should utilize CK-MB for this purpose)
- Presence of anti-cTnI antibodies can cause false negative test results
- Related laboratory tests include:
- Apolipoprotein A and B
- Aspartate aminotransferase
- Atrial natriuretic peptide
- Blood gases
- B-type natriuretic peptide
- C-reactive protein
- Creatine kinase and isoenzymes
- Electrolytes
- Lactate dehydrogenase and isoenzymes
- Lipid profile
- Lipoprotein electrophoresis
- Myoglobin
- Troponin T
Consult your laboratory for their normal ranges as these will probably vary somewhat from the ones listed below.
| Conv. Units
(ng/mL) | SI Units
(µg/L) |
|---|
| Normal | <0.4 | <0.4 |
| Suggestive of cardiac injury | >1.5 | >1.5 |
Conditions associated with elevated cTnI values include:
- Acute myocardial infarction (AMI)
- Acute coronary syndrome (ACS) or unstable angina
- Myocardial damage after:
- Coronary artery bypass graft surgery
- Other cardiac procedures
- Percutaneous transluminal coronary angioplasty (PTCA)
- Cardiac contusion or trauma
- Cardiac amyloidosis
- Defibrillation
- Nonischemic dilated or hypertrophic cardiomyopathy
- Myocarditis
- Radiofrequency ablation
- Non-cardiac causes:
- Cardiotoxic drugs
- b-Sympathomimetics
- Anthracyclines
- Antineoplastics
- Doxorubicin