|Values are valid only on day of printing.|
Exclusion diagnosis of acute myocardial infarction
Monitoring acute coronary syndromes and estimating prognosis
Possible utility in monitoring patients with nonischemic causes of cardiac injury
Troponin T is the cardiac marker of choice for the Mayo Health System for the evaluation of patients with possible cardiovascular injury
Troponin T is a myofibrillar protein found in striated musculature. There are 2 types of myofilament, a thick filament containing myosin and a thin filament consisting of 3 different proteins: actin, tropomyosin, and troponin. Troponin is itself a complex of 3 protein subunits: troponin T, troponin I, and troponin C. Troponin T binds the troponin complex to tropomyosin. Troponin I inhibits actomyosin ATPase in relation to the calcium concentration. Troponin C, with its 4 binding sites for calcium, mediates calcium dependency.
In the cytosol, troponin T is found in both free and protein-bound forms. The unbound (free) pool of troponin T is the source of the troponin T released in the early stages of myocardial damage. Bound troponin T is released from the structural elements at a later stage, corresponding with the degradation of myofibrils that occurs in irreversible myocardial damage. The most common cause of cardiac injury is myocardial ischemia, ie, acute myocardial infarction. Troponin T becomes elevated 2 to 4 hours after the onset of myocardial necrosis, and can remain elevated for up to 14 days.
Elevations in troponin T are also seen in patients with unstable angina. The finding of unstable angina and an elevated troponin T are known to have adverse short- and long-term prognoses, as well as a unique beneficial response to an invasive interventional strategy and treatment with the newer antiplatelet agents and low-molecular-weight heparin.
Values > or =0.01 ng/mL have been shown to have prognostic value.
The upper limit for normal individuals is <0.01 ng/mL (undetectable by this method).
For patients who present with acute coronary syndromes, troponin T values > or =0.01 ng/mL that are rising make the diagnosis of cardiac injury. Decreasing values are indicative of recent cardiac injury.
Troponin T values > or =0.01 ng/mL are a prognostic sign in patients with ischemic heart disease and most other situations. Clinical judgment is necessary to distinguish patients who have ischemic heart disease from those who do not. However, all patients with > or =0.01 ng/mL troponin T are at increased risk for cardiac events relative to patients with undetectable troponin T.
Patients with low level (<0.20 ng/mL) elevations of troponin T and diagnostic uncertainty for acute coronary syndrome should be evaluated by repeat measurements at 3 and 6 hours including a delta between these time points to determine whether this is an acute or more chronic elevation. However, all patients with > or =0.01 ng/mL troponin T are at increased risk for cardiac events relative to patients with undetectable troponin T.
As with all markers of cardiac injury, elevations of troponin T do not in and of themselves indicate the presence of an ischemic mechanism. Many other disease states are associated with elevations of troponin T via mechanisms different from those that cause injury in patients with acute coronary syndromes. These include trauma (eg, contusion, ablation, or pacing), congestive heart failure, hypertension, hypotension (often with arrhythmias), pulmonary embolism, renal failure, and myocarditis.
Various cutoffs have been proposed to identify patients with acute coronary syndromes and those at risk for future cardiac events. The cutoff used in earlier clinical studies (receiver operator curve cutoff) was around 0.10 ng/mL troponin T. This cutoff misses many patients at risk for short- or long-term adverse outcomes. The cutoff previously used at Mayo Clinic, 0.03 ng/mL, was based on the analytical performance of existing methodology for troponin T, and was chosen to approximate the level that could be measured with <10% imprecision. More recent recommendations suggest that a level of troponin T that exceeds the 99th percentile of values found in a healthy population optimizes identification of patients at risk of cardiac events. Levels seen in the normal population are undetectable (<0.01 ng/mL) by the current troponin T assay.
Jaffe AS: 2001-A biomarker odyssey. Clin Chim Acta 1999;284:197-211