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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