Friday, 26 February 2016

Does an undetectable troponin upon presentation to the ED rule out MI?

Spoiler alert: Probably yes if they have had pain for a couple of hours.

There have been many consistent studies proving the high sensitive troponin assays become positive very early after myocardial infarction. Therefore one can presumably use it in a rapid rule out MI strategy. Unfortunately the international guidelines have yet to catch up with the science.

Along comes another study with a slightly different spin that shows one can exclude MI early using a high sensitive troponin assay showing an undetectable result. This was a single centre study done in an Australian ED looking at an “undetectable” troponin at presentation and whether this could exclude MI. The thought being this could hopefully result in early discharge without further observation.

Once again, this was not a troponin simply below the usual reference range of normal but it was below the detectable threshold of the assay. Of course one could reasonably be concerned this will crush the specificity of the test- we’ll get to this in a moment.

The primary outcome was a final diagnosis of MI in patients with undetectable troponin at presentation to the ED. (They also included some risk stratification with TIMI scores, but feel free to ignore this as it was not helpful).

Results? The undetectable troponin was 97% sensitive (95% CI 93-99%) out of the 1076 patients studied. There were four patients missed by this strategy. A post hoc analysis (a reasonable data dredge) showed that an undetectable troponin was 100% sensitive in those who had pain for at least 2 hours before presentation to the ED. Of course this makes sense and is consistent with all of the other literature on the subject.  

Interestingly, I was quite surprised to see 60% of the patients in this cohort had an undetectable troponin upon presentation. I would have thought this number would have been much worse… goes to show what I know.

In the end, this study really is just confirming what we already know about the new high sensitive troponin assays. They are very good at picking up MI’s early after onset of infarction. Almost all patients who rule-in will do so within a few hours of symptoms. The days of the six hour troponin are probably gone. Come on guidelines… catch up please!



Covering:

Kelly A-M, Klim S. Does undetectable troponin I at presentation using a contemporary sensitive assay rule out myocardial infarction? A cohort study.  Emerg Med J 2014;0:1-4.



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