🔬 Troponin

Troponin is a protein found in cardiac muscle cells, released into the bloodstream following myocardial injury. High-sensitivity troponin assays (hs-cTn) are the cornerstone of diagnosing myocardial infarction (MI) in the UK. They are used within specific temporal algorithms to 'rule in' or 'rule out' ACS. Interpretation requires differentiating between acute ischaemic injury and non-ischaemic causes of elevation.

Troponin testing is primarily indicated in patients presenting with clinical features suggestive of acute coronary syndrome (ACS), such as central crushing chest pain, radiation to the jaw or left arm, or unexplained breathlessness. It is also used to assess myocardial strain in patients with suspected pulmonary embolism, acute heart failure, or severe sepsis. Routine screening in asymptomatic patients is not recommended.

Troponin T or I is measured via high-sensitivity immunoassay using a venous blood sample (yellow or orange top tube). These assays detect minute quantities of protein released from damaged myocytes. The 'high-sensitivity' nature allows for rapid rule-out protocols by detecting levels well below what was possible with older assays. Ensure the laboratory-specific reference ranges and 99th percentile upper reference limits (URL) are used.

A normal finding is a troponin level below the laboratory's 99th percentile upper reference limit (URL). In many UK trusts using high-sensitivity Troponin T, this is often <14 ng/L, though thresholds are gender-specific in some assays. A stable, low-level result on serial testing effectively rules out acute myocardial necrosis.

High-sensitivity troponin (hs-cTn) assays are interpreted using 'rule-in' and 'rule-out' algorithms, typically at 0/1 hour or 0/3 hours. A single low level at presentation in a patient with symptoms lasting >6 hours may rule out MI. A significant delta (change) between serial samples suggests an acute event rather than chronic elevation. Clinical context is vital; an elevated troponin is not synonymous with a 'heart attack' (plaque rupture).

Elevated troponin (above the 99th percentile URL) indicates myocardial injury. In the context of ischaemic symptoms or ECG changes, a rising or falling pattern is diagnostic of myocardial infarction (NSTEMI or STEMI). Chronic elevations may be seen in stable heart failure, chronic kidney disease, or structural heart disease. Very high levels are often seen in massive pulmonary embolism, myocarditis, or Takotsubo cardiomyopathy.

Troponin is the gold standard biomarker for diagnosing acute coronary syndromes (ACS). It allows for the stratification of patients presenting with chest pain into low-risk (suitable for discharge) and high-risk (requiring invasive coronary angiography) categories. Clinicians must distinguish between Type 1 MI (plaque rupture) and Type 2 MI (supply-demand mismatch, e.g., tachycardia or anaemia), as management strategies differ significantly.

Common pitfalls include assuming every elevated troponin requires dual antiplatelet therapy (DAPT) and anticoagulation; this is only for Type 1 MI. Misinterpreting chronic stable elevations in CKD as acute ACS is a frequent error. Also, failing to repeat the test in patients with very recent symptom onset may lead to missing a 'late riser' MI.

Troponin identifies myocardial necrosis but does not specify the underlying mechanism. False positives for ACS can occur in renal failure due to reduced clearance, or in pulmonary embolism and myocarditis due to non-ischaemic strain. Very early presentation (within 1-2 hours of symptom onset) may result in an initial negative result even in the presence of an evolving MI, necessitating serial testing.

High-yield for MLA: Understand the 4th Universal Definition of MI. Be able to differentiate between MI and myocardial injury. Recognise that a 'troponin-positive' patient requires an ECG and careful history to determine the cause. Knowledge of the 0/3 hour protocol is essential for UK foundation practice.