Rapid Protocols to Rule out Myocardial Infarction.

Evidence Summary Cardiac troponin T and I are released into the bloodstream when cardiac muscle is damaged. Cardiac troponin tests have been available for decades and are the preferred biomarkers for the diagnosis of acute myocardial infarction (AMI).1 However, until recently, they lacked sensitivity in the first few hours following an acute myocardial injury. Therefore, AMI could not be ruled out until the patient had a normal troponin test result at least six hours after the onset of chest pain.2 Recently, several companies have developed novel high-sensitivity troponin assays. They include a high-sensitivity troponin T (hsTnT) assay and a high-sensitivity troponin I (hsTnI) assay. Because the interpretation of tests differs, it is important for physicians to know which test is being used at their institution and the test’s lowest detectable level. Researchers have begun to explore protocols with the potential to rule out AMI earlier, so that patients can be discharged within a few hours of arrival rather than six to 12, or even 24, hours later. These protocols have used hsTnT3-6 and hsTnI.7,8 There are two general approaches. The first identifies the lowest identifiable level of troponin, and AMI is ruled out in any patient with an undetectable level of troponin on arrival at the ED.3,7,8 The second identifies patients with an initial low troponin level and no more than a small increase in levels between arrival at the ED and a second measurement one to two hours later.4,5 All of these protocols excluded patients with an obvious cause of chest pain such as vehicular trauma or who had ischemic changes on electrocardiography (ECG). To be useful, a test should identify a substantial group of patients with a very low likelihood of AMI or cardiovascular death in the next 30 days, to allow outpatient evaluation of the patient’s chest pain. Protocols that rule out a larger percentage of patients are preferable to those that rule out only a few. Table 1 summarizes the most recent evidence regarding protocols to rule out AMI, by type of test and the percentage of patients in each study meeting the rule-out criteria.3-8 Studies ranged in size from 270 to 14,636 patients and evaluated the ability to rule out AMI and determine 30-day mortality. Among protocols using the hsTnI assay, the most useful used a ruleout criteria of no AMI on the initial ECG and an initial hsTnI level of less than 5 ng per L; 46.1% of patients met these rule-out criteria.7 The study was large, with more than 6,000 patients, and only 0.4% (one in 250) had an AMI or cardiac death in the 30 days after discharge from the ED. Among protocols using the hsTnT assay, the one identifying the largest percentage of patients as low risk used both a low initial hsTnT and no more than a small increase in that value over the next one to two hours.4,5 These were defined as an initial troponin T value less than 14 ng per L that increases by less than 4 ng per L in the next two hours, or an initial level less than 13 ng per L that increases by less than 3 ng per L in the next hour. Another promising strategy simply identified patients with an initial hsTnT level of less than 5 ng per L as the ruled-out group; only 0.17% of these patients had a final diagnosis of AMI, and only 0.02% died within 30 days.3 Point-of-Care Guides