The new test further supports efforts to redefine heart attack, researchers say.
According to National Center for Health Statistics estimates, almost 1.5 million people are hospitalized each year with unstable angina (chest pain) and an electrocardiogram irregularity called non-ST segment elevation heart attack, which is indicative of a “mild” heart attack. Both are life-threatening conditions that require urgent care and hospitalization.
The American College of Cardiology and the European Society of Cardiology have recently recommended that any elevation of troponin – a protein produced by dying heart cells – should be considered a heart attack. However, determining the effects of this new standard are difficult, because of the number of different troponin tests and different elevation cutoff points used, says Michael Kontos, M.D., assistant professor of internal medicine and cardiology at Virginia Commonwealth University in Richmond.
Researchers evaluated different blood test markers in 4,117 patients with chest pain admitted into the hospital’s coronary care unit. They compared the predictive value of the standard heart attack marker, the enzyme creatine kinase (CK-MB), to a form of troponin known as troponin I. Both are used to diagnose heart attack in people having chest pain.
CK-MB is released not only by damaged heart tissue but other damaged tissues, which can cloud results. Furthermore, when a patient suffers a mild heart attack, CK-MB levels may remain normal, making diagnosis difficult. Troponin, however, is a protein produced by dying heart cells, and prior studies have found that its presence may indicate that heart damage has occurred.
The overall incidence of heart attack when CK-MB was used as the marker was 8.3 percent. Researchers also compared the cutoff points of troponin in two levels. When the upper reference value for troponin I was used, the number of heart attack patients increased by a relative 22 percent. In contrast, when the lower limit of detectability for troponin I was used, the number of patients with heart attack increased by a relative 160 percent, says Kontos.
“Evaluating patients with symptoms suggestive of a heart attack can be difficult when an electrocardiogram is inconclusive,” says Kontos. “Over a four-year period, we have found troponin at lower levels is more sensitive for diagnosis of heart attack than CK-MB alone in such patients.
“The troponin test has significant predictive value, suggesting that the number of mild heart attacks in such patients is far greater than we have been able to recognize in the past,” he says.
According to Kontos, the manufacturers of the many available blood tests for troponin use higher cutoff points than the researchers found for confirming heart attack.
“Our study provides a more sensitive method for identifying patients with heart damage,” he says. “Depending on the diagnostic value used for the tests, the troponin standard could more than double the number of patients who have had a non-ST elevation heart attack.”
Normally the level of troponin I in the blood is very low, and is usually undetectable. It increases substantially within several hours (on average four to six hours) of heart muscle damage. It peaks at 10 to 24 hours and can be detected for a week or more after.
Several studies have identified a measurable relationship between cardiac troponin levels and long-term outcome after an episode of chest discomfort. They suggest that these tests may especially help evaluate levels of risk.
“In other words, it’s possible that the results of a troponin test could be used to identify people at risk for later, serious heart problems,” says Kontos.
Troponin tests have been increasingly used throughout hospitals in the United States because of their recognized high sensitivity for identifying small amounts of heart damage. A revised definition of heart attack could have significant implications on health care since patients who in the past would have been labeled as having unstable angina will now be identified as having had a heart attack, says Kontos. It could make it difficult to compare heart attack rates, outcomes and treatment rates between physicians and hospitals.
“The use of troponin tests is being incorporated into several new American Heart Association/American College of Cardiology guidelines that deal with professional recommendations for the management of patients with heart disease,” says Sidney Smith, M.D., chief science officer of the American Heart Association. “We believe that the improved sensitivity of this test will allow us to better identify and risk-stratify patients for early therapy and thus improve outcomes.”
The study’s co-authors are: M. Lucie Fritz; F. Philip Anderson, Ph.D.; Joseph P. Ornato, M.D.; James L. Tatum, M.D.; and Robert L. Jesse, M.D., Ph.D.
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Abstract 3346