A new type of blood test has been found to safely and quickly rule out heart attack, as well as identify patients who are at low risk of suffering a heart attack in the near future.
In an observational study of patients with suspected heart attack admitted to the emergency departments of 15 U.S. medical centers, two high-sensitivity tests for troponin T (hs-TnT) taken three hours apart accurately identified those who were not having a heart attack and could be safely sent home.
The authors of the study, published online in JAMA Cardiology Dec. 13, 2017, said routine use of hs-TnT would allow appropriate patients to be discharged quickly and safely, freeing up emergency personnel to care for other patients with real emergencies.
Detecting a Heart Attack. When an artery is blocked, injured heart muscle cells spill proteins into the blood. These proteins, known as troponin T and troponin I, are usually kept inside the cells. When they are released, it’s a sign the cells are beginning to die.
Despite the classic image of a heart attack patient experiencing severe chest pain, many heart attack symptoms are less dramatic and nonspecific. Shortness of breath and nausea, for example, can be caused by many different conditions. This makes an positive diagnosis of heart attack difficult by symptoms alone.
Anyone who experiences a symptom consistent with heart attack is encouraged to seek immediate medical care, but only about 10 percent are actually having a cardiac event. Often times, a blockage will occur, disappear, and recur, causing blood flow—and symptoms—to start and stop over and over again. These patients are said to have acute coronary syndrome (ACS), or an incomplete heart attack.
Standard blood tests for troponin T are not sensitive enough to identify patients with ACS, who are at increased risk of having a heart attack in the near future. The hs-TnT test is 1,000 times more sensitive. Its value lies in its ability to identify patients who are having a heart attack, but also those who are unlikely to have a heart attack within 30 days. This gives physicians the confidence to release ACS patients in a few hours, rather than admitting them to the hospital or keeping them under observation for at least 23 hours, as is common practice today.
Testing for Accuracy. The hs-TnT test is not new in Europe, but only became available in the U.S. in January 2017. U.S. researchers wanted to see whether the blood level of elevated TnT that signaled a heart attack was the same in U.S. patients as in European patients. As it turned out, it was not.
By using the blood test on 1,301 healthy volunteers, they discovered that 99 percent of people who are not having a heart attack have a troponin T level of 19 ng/dL or less. In Europe, this cutoff point is 14 ng/dL.
By testing 1,600 patients with symptoms of ACS, they found that a single hs-TnT level less than 6 ng/dL upon admission to the ED ruled out heart attack with 99.4 percent accuracy. A second hs-TnT test three hours after admission further predicted these patients would not have a heart attack within 30 days with 99.3 percent accuracy.
The test was not 100 percent accurate, however. Seven patients given the all-clear signal within three hours had ACS. Fortunately, none suffered a heart attack or died. The researchers concluded that two hs-TnT tests given three hours apart would be a clinically useful and acceptable method of ruling out heart attack.
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