Wednesday, 10 September 2014

Can We Rule-out MI in Two Hours?

I love any study that provides evidence that allows us to use less time or resources in the Emergency Department. Can I use a high-sensitive troponin (hsTnT) assay to rule-out MI in two hours rather than the traditional six? Yes, but with some cautions and caveats.

The authors of this paper looked to validate an accelerated hsTnT assay measured at 0 and 2 hours for diagnosing acute myocardial infarction in the Emergency Department.

This was a prospective observational cohort study conducted on consecutive eligible patients at the Royal Brisbane and Women’s Hospital in Australia. Inclusion criteria specified adults whereby the treating doctor thought they should perform serial troponin measurements to rule-out MI. In addition to the hsTnT assay they also got all of the usual work-up for potential ACS including ECG’s, “old school” troponin I at 0 & 6 hours and other follow up.

As with all studies investigating a new “better” test, the gold standard is always a little troublesome. To what do you compare your new test against when yours is presumably the best? This study used the usual adjudicated diagnosis by a non-blinded cardiologist based on all of the clinical data available. I do not believe they were blinded to the hsTnT as this was not 100% clear in the text.

What were the results? 764 patients were enrolled. There was a 7.3% (56 patients) rule in MI rate which is rather low compared to usual Australian standards but probably high compared to those in North America. The hsTnT measured at 0 & 2 hours was 96% sensitive and had a negative predictive value of 99%. Only two patients out of 56 were missed with this strategy. Sounds great! But some caution is in order.

The median time from symptom onset to presentation was FIVE hours. Therefore be very careful in patients who present early after chest pain onset. Other studies (Reichlin or Keller NEJM August 27, 2009) have shown that the hsTn assays generally become positive within about three hours. So it is not surprising that the early hsTnT worked well in this study. In fact the sensitivity of the first troponin was 93%. The addition of the two hour troponin only picked up two more patients and increased the sensitivity to 96%.

Don’t get overly excited about the 99% negative predictive value as this of course is a reflection of the low prevalence of disease in this cohort. Remember they started out with a negative predictive value of 93% before any test was performed. This is bound to be worse in a higher risk cohort.

The authors go on to state that time saved by this approach could have a major impact on health service delivery. Not so fast. I agree that two hours is better than six. But the high sensitive troponin assays have already added to our work load with its drop in specificity. More and more patients are “positive” creating a big headache. In the end, I have no doubt they increase further diagnostic testing and admission rates. So I doubt we can consider the high sensitive troponin assays as time or resource saver.

What do I take home from this? If my goal is to rule-out MI, I can probably do this with adequate sensitivity with a 0 and 2 hour  high-sensitive troponin in a patient with reasonably low pre-test clinical suspicion. But not in a patient who is high risk or presents early after chest pain onset. In the end, I am very happy that there is now some more evidence that can support an accelerated rule-out MI strategy in suitable patients.  



Covering:

Parsonage WA, Greenslade JH, Hammett CJ, et al. Validation of an accelerated high-sensitive troponin T assay protocol in an Australian cohort with chest pain. Med J Aust 2014;200(3) 161-165.



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