Rapid NSTEMI exclusion becoming reality

By Eleanor McDermid

Two studies show that modern high-sensitivity troponin I (hsTn) assays can help physicians to rapidly identify patients with a very low likelihood of having non-ST-segment elevation myocardial infarction (NSTEMI).

Both studies are published in JAMA Cardiology. One evaluates a 1-hour protocol - testing patients at presentation and 1 hour later - while the other focuses on identifying a cutoff that, along with a normal electrocardiogram (ECG), can exclude acute MI (AMI) at the time of presentation.

David Morrow (Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA) writes in an editorial that the studies "lend strong support to the notion that accelerated diagnostic protocols that incorporate hsTn can facilitate earlier triage while maintaining an acceptable NPV [negative predictive value]."

He notes that the limit of detection seems to be the best cutoff "to deliver an acceptable NPV at presentation."

The first study, from Edward Carlton (Southmead Hospital, Bristol, UK) and colleagues, involved 3155 patients with a non-ischaemic ECG, of whom 18.8% had Tn levels below the limit of detection (1.2 ng/L) and 9.2% developed AMI.

The sensitivity for AMI was 99.0% and the NPV was 99.5%. None of the three patients with a false-negative result died within 30 days, the team notes. At higher thresholds, from 2.0 up to 5.0 ng/L, the sensitivity fell below the researchers' minimally clinically acceptable threshold of 99.0%, and NPV fell below 99.5%, although it remained above 99.0%.

The reduced performance at higher thresholds was particularly marked in the 1047 patients who presented within 2 hours of symptom onset. Sensitivity was also lower at the limit of detection in these patients, at 98.6%.

Given this, "[i]t seems prudent to obtain a sample at 3 hours or later in early presenters (or ambiguous timing) and in patients with clinical high-risk features", says Morrow, in his editorial.

In the second study, Dirk Westermann (University Hospital Hamburg Eppendorf, Germany) and team tested a 1-hour triage algorithm in 1040 patients with non-ischaemic ECGs and validated it in a further 4009.

They found 6.0 ng/L to be the optimal Tn cutoff, which if applied only at presentation, gave an NPV for AMI of 97.1%. This increased to 99.0% and 99.5% if repeated after 1 and 3 hours, respectively, with no statistical difference between the two. All NPVs were higher for NSTEMI type 1, at a corresponding 98.5%, 99.8% and 100%.

Like Carlton et al, the researchers found the NPV decreased with higher cutoffs, and they saw an associated 12-month mortality increase, with a rate of 1.0% for the 6.0 ng/L cutoff but 3.7% for the "routinely used" 99th percentile (27.0 ng/L).

Morrow stresses that Tn tests should always be used in conjunction with clinical assessments and also highlights that physicians should be aware of "the performance both of your [cardiac] Tn assay and in your population."

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