High Sensitivity Troponin T (hsTnT): Testing & Interpretation
Troponin testing has been extensively validated as a prognostic marker in acute coronary syndromes. It does not replace history, examination and commitment to a differential diagnosis in a prospective way.
Principal indications for measurement:
- Risk Stratification in ACS (see GRACE risk score and Trust chest pain guidelines) in conjunction with history and ECG Confirmation and extent of MI in conjunction with history, ECG and imaging
It is also frequently measured and elevated in:
- Myocardial injury (reversible or irreversible) due to non-coronary disease
- As with all laboratory tests there is a very small risk of laboratory false positives/negatives. Note also that using different sample types will give slightly different results and this may cause confusion when assessing %changes between sequential samples. For this reason only serum is accepted for hsTroponin T analysis at GHNHSFT.
Use the Troponin Flow Chart to determine which of the following diagnostic groups is most likely:
DIAGNOSTIC GROUP 1
Risk stratification in ACS
This assumes coronary artery disease and atheromatous plaque rupture as the underlying aetiology. Treatment is therefore directed at this with antiplatelets and anticoagulants, followed by secondary prevention and usually invasive imaging/intervention.
A negative troponin result DOES NOT exclude a diagnosis of ACS, but places the patient in a lower risk category. They may still require urgent intervention.
Confirmation and extent of MI
This is usually due to atheromatous plaque rupture. Consider also embolisation and dissection. A rise of >100% between 1st and 2nd troponin is more likely to indicate infarction.
In STEMI immediate referral to a catheter laboratory is mandatory. DO NOT WAIT for a troponin result.
DIAGNOSTIC GROUP 2
Myocardial injury due to non-coronary disease
This is a false positive for ACS in which serum troponin is actually elevated, but due to myocardial stress from other cardiac causes. These include:
Myocarditis, arrhythmias, non ischaemic acute LVF, PE, contusion (eg RTA or other trauma), electrocution (including DC cardioversion), infiltrative cardiac disease, etc.
ACS treatment with antiplatelets and anticoagulants is unhelpful in the absence of atheromatous plaque rupture AND MAY BE CONTRAINDICATED.
DIAGNOSTIC GROUP 3
Non cardiac diagnosis with severe systemic compromise
This is a false positive for ACS in which serum troponin is actually elevated, but due to myocardial stress from serious systemic illness. E.g.:
Sepsis, non-cardiogenic shock, anaemia (e.g. GI bleed), hypoxia, metabolic and endocrine derangement (e.g. renal failure, acidosis, thyrotoxicosis, myxoedema), cerebrovascular accidents, etc.
A rise of <20% between 1st and 2nd troponin is likely to indicate non-ischaemic aetiology.
ACS treatment is CONTRAINDICATED.
In Groups 2 and 3 troponin elevation remains an adverse prognostic indicator by virtue of the severity of the underlying diagnosis. Admission to an APPROPRIATE high dependency area (often not CCU) saves lives.
DIAGNOSTIC GROUP 4
Laboratory False Positives / Negatives
This is a true false positive where the serum troponin is NOT actually elevated e.g. due to analytical interfering factors, technical errors or analyser malfunction.
Analytical interfering factors include: antibodies (RhF, autoantibodies, heterophile antibodies), immunocomplexes, fibrin clots (anticoagulant therapy, coagulopathy), serum bilirubin, haemoglobin, microparticles.
The Chemical Pathology analysers at CGH and GRH routinely check for haemoglobin (haemolysis) bilirubin (icterus) and turbidity (lipaemia) in samples assayed for Troponin T and a result will therefore not be reported where these are present in a high enough concentration to cause interference in the assay.
In rare cases, interference due to extremely high titres of antibodies to analyte-specific antibodies, streptavidin or ruthenium can occur. Samples should not be taken from patients receiving high dose Biotin until at least 8 hours following the last biotin administration.
It is possible to re-analyse samples and assess for such false positives when the result is incongruous with the patient history. Please contact the duty biochemist to discuss these cases.
CATEGORIES 1 TO 4 CAN ONLY BE DIFFERENTIATED BY ACCURATE HISTORY TAKING AND PROSPECTIVE DIAGNOSIS
Author: Dr S Siedlecka