The Use of Troponin for Diagnosis of ED Patients With Chest Pain
ABSTRACT & COMMENTARY
Source: Hamm CW, et al. Emergency room triage of patients with acute chest pain by means of rapid testing for cardiac troponin T or troponin I. N Engl J Med 1997;337:1648-1653.
Hamm and associates studied 773 patients presenting to the ED with acute chest pain and without ST segment elevation in a study from Hamburg, Germany. Diagnostic testing included measurements of troponin T and troponin I as well as ECG and CK-MB. Measurements were done within 15 minutes of arrival and again at four hours. Patients who presented to the ED less than two hours after the onset of chest pain were tested a third time at six hours following the onset of pain. The study end points were nonfatal acute myocardial infarction (MI) during hospitalization or after discharge and death from cardiac causes.
The purpose of this study was to determine the diagnostic and prognostic value of rapid bedside troponin testing for "early triage" in the ED. However, the study design was more consistent with short-term observation, in that all patients were observed for the duration of repeated diagnostic measurements. The troponin-T test was a whole-blood bedside assay, but the troponin-I test was performed on serum in a separate laboratory. Once the sample was processed, the serum obtained, and the test started, the troponin-I test took 15 minutes.
The results of this study are similar to other studies of troponin T and troponin I, in that both troponin tests are sensitive for myocardial injury, and the sensitivity increases with repeated testing. Initial testing was not useful as a screen. Among patients positive for troponin at any point, only 58% of those with positive troponin T were positive on arrival in the ED; of those with a positive troponin I, 64% were positive on arrival.
In those patients with acute MI without ST elevation, 47 patients were identified among the 773 in the study. Of these 47, 51% were positive for troponin T on arrival, and 94% were positive four hours later. By comparison, 66% had a positive troponin I on arrival, and 100% were positive four hours later.
The study also identified 315 patients with unstable angina. Of those, 22% had at least one positive troponin-T, 36% had at least one positive troponin I, and only 5% had a positive CK-MB. Therefore, the superiority of troponin to CK-MB in detecting minimal myocardial injury is confirmed in this study.
Follow-up of all patients in the study revealed that there were 20 deaths related to cardiac disease or sudden death, four of whom had negative results on all troponin-T tests, and one with negative results on all troponin-I tests. Thus, the negative predictive value for death in this population was 98.9% for troponin T and 99.7% for troponin I. One patient with negative results on all troponin tests had an acute MI within two weeks of discharge from the hospital.
Hamm et al conclude that troponin measurements are superior in predictive value to CK-MB and ECG, and that these diagnostic tests should be made available to EDs and chest pain units.
COMMENT BY JEFFREY W. RUNGE, MD, FACEP
The value of this study to emergency physicians over many previous studies is that it took place in the setting of the ED with acute chest pain patients. It confirms that troponin tests are more sensitive for minimal myocardial injury than CK-MB or ECG. Troponin testing does not, however, enable us to triage chest pain patients to admission or discharge after a single evaluation. Troponin testing is not useful as a screening tool as a single measurement shortly after the onset of chest pain. Neither is anything else.
The advent of more sensitive and specific tests for myocardial injury and the need to decrease expensive coronary care unit days for undifferentiated chest pain has led to a proliferation of chest pain evaluation centers around the country. This approach makes perfect sense when one considers that the sensitivity of testing increases over a finite period of observation. Thus, tests with high-negative predictive values are employed over the first 12 hours following the onset of pain to rule out myocardial cell damage. Following that, sensitivity is further increased by provocative testing to put the heart in a situation of increased myocardial oxygen demand. It is the sensitivity of the test and its negative predictive value that are important in the "ruling-out" of disease and which provide protection of patient and physician.
Using any combination of tests with less than a 100% negative predictive value in a disease with fatal consequences is less than ideal. The public has not afforded emergency physicians an acceptable failure rate when predicting who can be sent out of the ED with chest pain. The public, speaking through the legislative and judicial systems, has zero tolerance for error in this regard, and we must, therefore, be vigilant in our pursuit of this diagnosis. Troponin testing, ECG measurements with continuous ST-segment monitoring, and provocative testing are all mainstays of the evaluation of patients in whom cardiac disease is clinically suspected. So, we shall stay the course and take advantage of slow progress in the diagnosis of chest pain. There is no magic test for "early triage" and no substitute for clinical judgment and the rational application of diagnostic testing over an appropriate period of time.
Troponin T and I testing:
a. is more sensitive for minor myocardial injury than is CK-MB.
b. is most useful as a single-measurement screening tool in the ED triage of chest pain.
c. may be positive in acute MI, but is not positive in unstable angina.
d. is invalid in patients with chronic renal failure.
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