As we’ve seen suggested in prior work, an undetectable high-sensitivity, high-precision troponin at presentation is a powerful predictor of minimal risk in the short-term.
In this prospective study based at four hospitals in Scotland, 6,304 eligible patients with suspected acute coronary syndrome were captured for analysis across three prospectively gathered cohorts. These patients were split across a derivation cohort to establish a threshold with an appropriate negative predictive value, an internal validation cohort, and an external validation cohort. Their target for a 30-day MACE of MI or cardiac death was a negative predictive value of 99.5%.
Making essentially a long story short, they found a threshold of 5 ng/L established this minimal-risk population, with only 9 patients meeting the primary outcome out of of 2,302 patients with an initial troponin less than the threshold. And, if you want to be even more airtight, 6 of these presented within 2 hours of the onset of chest pain, and 3 had apparent ischemic changes on their initial EKG – which may have been picked up by a rapid repeat testing protocol in the ED for some patients.
There are a few holes in this paper, of course. Not all patients were hospitalized and had such repeat testing, so some small nSTEMI or vasospasm events could have been missed. Patients in this study required a median of 54 minutes for blood sampling after presentation to the ED, so some caution should be exercised regarding repeat troponin testing if your department is efficient regarding phlebotomy at presentation and onset of symptoms was just prior to arrival. But, on the whole, this greatly adds to the body of evidence we’ve been building – that cutting edge troponin assays alone can provide powerful prognostic information.
Now, what to do with all the “intermediate” cases ….
“High-sensitivity cardiac troponin I at presentation in patients with suspected acute coronary syndrome: a cohort study”
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(15)00391-8/abstract
We have been using a high sensitive troponin assay in my regional Australian ED for almost 5 years now. There is no doubt it can be used for early rule out of MI and risk stratification.
However as Ryan cautions, one must still be careful in those patients that present to the ED very early after onset of pain. I have thrombolysed full blown STEMI's and had their first troponin absolutely negative when they come in early. The original two papers by Reichlin and Keller from the August 2009 NEJM demonstrated that patients generally ruled in by three hours.
In summary, still be careful with the early presenters. And consider repeating the troponin a little later.
Considering how frequently this test is ordered in Emergency Medicine, we ought to be experts on its test characteristics by now. Unfortunately, I think we still have a lot of folks treating it as a dichotomous test, and probably not necessarily realizing the sensitivity yet has limits. I think the protocol I developed here suggests a 2h delta if the pain was onset 2 or 4 hours prior to presentation.