Single Blood Test Rapidly Rules Out MI in Chest Pain

–New high-sensitivity troponin tests may ease perpetual ED dilemma

Only a small percentage of emergency department chest pain patients turn out to have acute myocardial infarction (AMI). Now a new blood test can help identify a significant number of patients who are extremely unlikely to have AMI and who can therefore be safely discharged immediately.

In January, the FDA cleared the first of the new generation of high-sensitivity cardiac troponin T (hs-TnT or hs-cTnT) blood tests for the rapid diagnosis of AMI. The test has not yet been made available in the U.S., but experts believe that it could have a profound effect on the diagnosis and treatment of AMI in the ED.

A new meta-analysis published in Annals of Internal Medicine contains a detailed review of the available evidence and shows that, by using the test in the ED, AMI “may be ruled out in a substantial proportion of patients after only one blood draw.” John Pickering and Martin Than, both from Christchurch, New Zealand, led a collaborative effort to identify and analyze prospective studies that looked at the use of hs-TnT in the ED. Out of 9,241 patients in 11 studies, 30% were classified as low risk because they had no new ischemia on ECG and had hs-TnT measurements lower than 0.005 μg/L. In this low risk group, only 14 patients (0.5%) had AMI within 30 days and none died.

The authors cautioned that because troponin may not be detectable in the first hours of MI, the test should be used with caution within 3 hours of symptom onset. They recommended obtaining a second sample after the third hour of symptoms. They also warned that the results are “assay specific” and may not apply to future products. In addition, there may be variation in manufacturer batches and results can depend upon “setup, calibration, and operation of analyzer in laboratories at individual sites.”

James De Lemos (UT Southwestern) commented that the meta-analysis “confirms the findings of the individual cohort studies on which it was based and provides clear evidence that, among low-risk individuals who present at least 3 hours after the onset of symptoms, a single measurement of hs-cTnT below the detection range effectively ‘rules out’ MI, without the need for additional testing, and might apply to as many as 30% of patients who undergo MI ‘rule out’.”

“This finding is important for U.S. physicians in light of the recent FDA approval of this assay. Other studies have shown that an additional 20% to 30% of patients might be able to be ruled out with serial measurements over a very short 1- to 2-hour period. Thus, as many as 50% to 60% of patients can be dispositioned rapidly, which may improve emergency room through-put and reduce crowding.”

De Lemos warned, however, that the positive benefits of the new test will need to be introduced carefully in the U.S. “When the hs-cTnT assay is rolled out in the U.S. it is absolutely imperative that it be combined with protocols for rapid MI rule out, in order to capitalize on the advantages of the assay.

“If hospitals simply replace the current assay with the more sensitive one, without changing their protocols, they will not capture these advantages and will deal with a host of potential disadvantages, as the assay is less specific than the current generation assay, and more likely to be elevated from conditions other than MI.”

The discussion continues:

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