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A protocol for exclusion of myocardial infarction using hs-cTnT
Summarized from Dawson C, Benger J, Bayly G. Serial high-sensitivity troponin measurements for the rapid exclusion of acute myocardial infarction in low-risk patients. Emerg Med J 2013; 30: 593-94.
Diagnosis of myocardial infarction in the absence of characteristic electrocardiographic (ECG) changes depends on measurement of circulating cardiac troponin (cTn); either cardiac troponin I (cTnI) or cardiac troponin T (cTnT).
By definition myocardial infarction is associated with increased serum cTnI and cTnT; typically a rise can first be detected, using high-sensitivity assays (hs-cTnI, hs-cTnT), from around 2-4 hours after the onset of symptoms. Concentration peaks at 12-24 hours and gradually returns to baseline (normal) concentration over the following week or two.
The absence of a rise (or fall) in cardiac troponin can be used to exclude a diagnosis of myocardial infarction, and it is this aspect of troponin testing that is addressed in a recently published study. The authors of this retrospective study sought to define a protocol to reliably exclude myocardial infarction as rapidly as possible, using serial hs-cTnT measurement.
They retrieved data relating to all patients who had hs-cTnT measurement in the emergency department over a 2-month period. All study patients had two measurements: one at admission and another at 8 hours after symptom onset (the standard protocol at this emergency department at the time of the study).
The time of onset of symptoms and final diagnosis for each study patient were recorded. Results were stratified to three groups: negative, <14 ng/L (upper limit of normal); indeterminate, 14-29 ng/L; and positive, ≥30 ng/L. The study focused on the subset of patients (334 in total) who had a negative or indeterminate hs-cTnT on admission.
The first major finding was that all patients presenting more than 3 hours after onset of symptoms who had an initial negative result (<14ng/L) also had a negative result 8 hours later. By definition, none of these patients had suffered myocardial infarction.
On the basis of these results, which are in line with many previous studies, the authors concluded that a negative result at 6 hours post symptom onset reliably excludes myocardial infarction. Accordingly they have changed their protocol so that the second sample is taken at 6 hours after onset of symptoms, rather than at 8 hours.
The second major finding of this study addresses the problem of how to reliably exclude myocardial infarction in patients whose hs-cTnT changes only minimally between first and second sampling.
A significant increase on serial testing, of course, indicates myocardial infarction, but the magnitude of the change required to reliably confirm the diagnosis, remains controversial. Analysis of study data revealed that among patients with an indeterminate level at admission, none had a final diagnosis of myocardial infarction if increase in hs-cTnT was less than 46.2 %, or in absolute concentration terms, less than 16.5 ng/L.
The authors suggest that their stratification approach provides a practical way of excluding myocardial infarction: ”coronary angiography need only be considered in patients with negative (<14 ng/L) or indeterminate (14-29 ng/mL) hs-cTnT results at presentation, if second result is positive (>30ng/L)”.
The final diagnoses for those study patients whose hs-cTnT was increased (>14ng/L) but had not suffered myocardial infarction is recorded in the study report – a reminder that there are many conditions other than myocardial infarction that can cause increase in circulating cardiac troponin.
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