Troponin I and troponin T (cardiac enzymes)

Cardiac Troponins

• Troponins are protein components of striated muscle. There are three different troponins: troponin C, troponin T and troponin I. Troponins T and I are only found in cardiac muscle
  • Troponin T (1)
    • 84% sensitivity for myocardial infarction 8 hours after onset of symptoms (1); 81% specificity (1)
    • low specificity - 22% for unstable angina
    • advantages
      • highly sensitive for detecting myocardial ischaemia
      • levels may help to stratify risk afterward
  • Troponin I
    • 90% sensitivity for myocardial infarction 8 hours after onset of symptoms (1); 95% specificity (1)
    • low specificity for unstable angina - 36% - note however that there is evidence that (2) troponin I elevation is useful for predicting in-hospital risk for unstable angina patients admitted to a community hospital. The association of ECG changes and high troponin I identifies a population at very high risk; however, the absence of both variables in patients with a diagnosis of unstable angina does not preclude the development of events
• rises after 3-6 hours (1)
• peaks at about 20 hours (1)
• general advantages (3)
  • troponin T (cTnT) and troponin I (cTnI) are released only following cardiac damage
    • CK and CK-MB are found in skeletal muscle as well as cardiac muscle - therefore if there is damage to skeletal muscle, elevations of CK and CK-MB will occur and can make the diagnosis of myocardial infarction difficult. In such a situation levels of cTnT and/or cTnI will not rise unless myocardial infarction has occurred
  • troponin T and I are present for, and remain elevated, a long time
    • unlike CK and CK-MB, cTnT and cTnI are released for much longer with cTnI detectable in the blood for up to 5 days and cTnT for 7-10 days following MI. This allows an MI to be detected if the patient presents late. For example, if a patient comes to the surgery with a history of chest pain 2-3 days ago, measurement of cTnT or cTnI will allow the diagnosis or exclusion of MI as a cause of the chest pain
  • troponin T and I are very sensitive
    • there is always a low level release of CK and CK-MB from skeletal muscle at a low level all the time so there is always a background value. This is not the case for the cardiac structural proteins such as cTnT and cTnI and therefore, they are very sensitive. Studies have revealed that about one third of patients admitted with unstable angina, in whom MI was apparently excluded by CK and CK-MB measurement, have raised levels of cTnT and cTnI. Follow up studies have revealed that these patients are at significantly greater risk of death, subsequent MI or readmission with unstable angina than patients who did not have detectable levels cTnT or cTnI
• general disadvantages (3)
  • elevation of cTnT or TnI is absolutely indicative of cardiac damage, but this can occur as a result of causes other than MI e.g. myocarditis, coronary artery spasm from cocaine, severe cardiac failure,cardiac trauma from surgery or road traffic accident, and pulmonary embolus can cause cardiac damage with an accompanying elevation of cardiac troponin(s)
  • failure to show a rise in cTnT or cTnI does not exclude the diagnosis of ischaemic heart disease
  • both cTnT and cTnI may be elevated in patients with chronic renal failure and indicate a higher long-term risk of death. They can be distinguished from changes due to myocardial infarction by repeating the tests. Myocardial infarction causes a rise and fall in cTnT or cTnI, but in renal failure the elevated levels are sustained
  • reference ranges may vary between laboratories and are dependent on methods of measurement used

Reference:

1. Ebell MH et al (2000). A systematic review of troponin T and I for diagnosing acute myocardial infarction. J Fam Pract, 49, 550-6.
2. Rev Esp Cardiol 2002 Feb;55(2):100-106 [Is Troponin I Useful for Predicting In-Hospital Risk for Unstable Angina Patients in a Community Hospital? Results of a Prospective Study. Bodi V V, Sanchis J, Llacer A, Graells ML, Llorca L, Chorro FJ, Insa LD, Navarro A, Plancha E, Cortes FJ, Ponce De Leon JC, Valls A
3. British Heart Foundation (Factfile 08/2003). What are cardiac troponins?