Acute coronary syndromes incorporate two conditions: Unstable angina, and Acute myocardial infarction (AMI), which is further differentiated into: ST elevation myocardial infarction (STEMI), which can be diagnosed using an ECG, and Non-ST elevation myocardial infarction (NSTEMI), which can only be reliably diagnosed using cardiac biomarkers
Cardiac troponin is the biomarker of choice for diagnosing or excluding acute myocardial infarction (AMI).
CMFT currently uses the high sensitivity troponin T assay from Roche Diagnostics (hs-cTnT).
In patients whose symptoms and/or signs lead treating clinicians to suspect a diagnosis of acute coronary syndrome, hs-cTnT should be measured at the time of admission or as soon as possible following the onset of symptoms.The test should be repeated 12 hours after the onset of the peak symptoms.
The lowest reportable level of hs-cTnT is 3ng/L (the ‘limit of blank’ or LoB)
The ‘normal range’ of hs-cTnT is <14n/gL (the 99th percentile in apparently healthy individuals)
Levels between the limit of blank (3ng/) and 14ng/L are normal and will be detected in more than half of apparently healthy individuals
If any patient has a hs-cTnT level >14ng/L they should have a second sample sent for hs-cTnT testing six hours later.
Haemolysis can cause a falsely low level of hs-cTnT
AMI must not be ‘ruled out’ using a haemolysed sample
Haemolysed samples cannot be used to quantify the rise and/or fall of hs-cTnT on serial testing (the ‘delta troponin’).
A positive hs-cTnT (>14ng/L) may still be used to risk stratify patients, even in the presence of haemolysis. However, the sample should still be repeated immediately in order to allow quantification of the rise and/or fall.
To ‘rule out’ AMI
In patients who present to hospital >12 hours after the onset of peak symptoms, a single hs-cTnT level <14ng/L can be used to rule out AMI. However: the clinician should be certain that the peak symptoms occurred >12 hours ago. If there is any doubt, repeat the test 12 hours after the latest symptoms.
In patients who present to hospital <12 hours after the onset of peak symptoms, or whose symptoms occur while in hospital: AMI can be ruled out if both hs-cTnT levels are <14ng/L
Patients with hs-cTnT levels >14ng/L may still have AMI ‘ruled out’ if they do not demonstrate a significant rise and/or fall (delta troponin) on serial testing (see below). ‘Ruling out’ AMI in these circumstances is a clinical decision and must be based on the clinical context, putting together all of the relevant clinical information for the individual patient. Review by a senior clinician with expertise in this area is recommended. In order to ‘rule out’ a rise and/or fall of hs-cTnT, serial samples should be taken six hours apart.
In patients who present for medical attention very late (>72h) after symptom onset, hs-cTnT levels may have returned to baseline, precluding the detection of a rise and/or fall of hs-cTnT. If there is ongoing suspicion of late-presenting AMI, additional investigation is recommended.
To ‘rule in’ AMI
Hs-cTnT cannot ‘rule in’ AMI. The result must be interpreted in the clinical context. A diagnosis of AMI can be established in the following circumstances:
A rise and/or fall of hs-cTnT to above 14ng/L in conjunction with at least one of:
- Symptoms of myocardial ischaemia
- New or presumed new ST-segment-T wave changes or new left bundle branch block in the ECG
- Development of pathological Q waves in the ECG
- Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality
- Identification of an intracoronary thrombus by angiography or post-mortem