BACKGROUND: Emergency departments triage more than 5 million patients with chest pain each year. Cardiac blood tests are used to identify patients at higher risk for MI or death. The bedside instrument in this study is a new approach to the immediate risk stratification of patients with symptoms suggestive of myocardial ischemia.
POPULATION STUDIED: The investigators enrolled 1005 patients older than 18 years with possible myocardial ischemia who presented to the emergency room of 6 US hospitals. Patients were excluded if electrocardiography showed ST-segment elevation or left bundle branch block. The average patient age was 51years; 51% were women; 14% had a previous MI; 21% had diabetes; 53% had hypertension; and 38% were current cigarette smokers. The researchers were able to provide complete results for 95% of the patients.
STUDY DESIGN AND VALIDITY: This was a prospective study looking at the prognostic value of a bedside instrument measuring cardiac enzymes at the point of care. Blood samples were obtained from patients at baseline and at 3 and 6 hours. If the patient was hospitalized, samples were obtained at 9, 12, and 16 to 24 hours. These were analyzed by the Dade-Behring Stratus CS STAT near-patient instrument, which assays myoglobin; creatine kinase, myocardial bound (CK-MB); and troponin I (cTnI) from a blood sample in 15 to 20 minutes. Two multimarker strategies (MMS) were defined: MMS-1 included all 3 markers, and MMS-2 included only CK-MB and cTnI. A strategy was considered positive if any of the markers was positive. The CS STAT assay was compared with the CK-MB result of the local hospital laboratory. All patients underwent both local laboratory testing and bedside testing, but treating physicians used only local CK-MB results in making management decisions. This study was well designed for comparing the prognostic value of the CS STAT values with conventional CK-MB values. However, a more useful outcome would be the utility of this bedside instrument in the diagnosis of MI. Also, whether such an instrument would lead to clinically significant changes in prognosis outside of special chest pain units is unclear.
OUTCOMES MEASURED: The rate of MI or death at 30 days was determined for patients with either a positive or negative MMS or CK-MB test result. Additional assessments included time from arrival to a positive test result and the relation of MMS status to the rates of MI, death, and revascularization at 30 days.
RESULTS: Testing at baseline (on initial evaluation in the emergency department) predicted death or MI within 30 days in 19% of patients with a positive MMS-1, 22% with a positive MMS-2, and 13% with a positive CK-MB result. Conversely, only 3% with a negative MMS-1 or MMS-2 and 6% with a negative CK-MB result died or had a MI at 30 days. With serial testing 55% of persons with an abnormal CK-MB result had a MI at 30 days, while serial MMS testing did not discriminate any better than the single test. The bedside 3-marker strategy was superior in predicting 30-day mortality; all 3 patients who died had a positive MMS-1 at baseline, while only 1 of 3 had an abnormal CK-MB on serial testing. An abnormal CK-MB result predicted revascularization within 30-days better than bedside testing using both single and serial measurements. After emergency department arrival, positive test results were obtained more quickly using the bedside instrument (MMS-1=2.5 hours, MMS-2=2.8 hours, LL=3.4 hours; P=.0001).
A single bedside multimarker test (myoglobin, troponpin I, and CK-MB) is more likely than a single CK-MB result to determine risk for MI or death within 30 days after an episode of chest pain. Serial CK-MB testing outperformed both single and serial MMS testing in predicting 30-day MI, need for revascularization, and combined outcome of MI or death. This single bedside test may be a useful adjunct to standard tests in identifying patients at higher risk for MI or death. However, it should not replace serial CK-MB testing in the diagnosis of MI.