Use of Troponin for Chest Pain Triage
Use of Troponin for Chest Pain Triage
ABSTRACT & COMMENTARY
Synopsis: The new rapid troponin tests, are highly sensitive for detecting myocardial injury in acute coronary syndromes.
Source: Hamm CW, et al. N Engl J Med 1997;337:1648-1653.
Measurements of cardiac specific contractile proteins, troponin T and I can detect minor cardiac injury that is superior to creative kinase MB measurements for predicting adverse events in patients with chest pain. Recently, tests for troponin have been developed that provide rapid results (< 20 min), which could speed up emergency room triage of chest pain patients.Thus, Hamm and colleagues studied 773 consecutive patients with chest pain but without ST elevation on ECG or obvious evidence of chest trauma. Troponin T and I tests were performed on arrival and four hours later. In patients who presented within two hours of chest pain onset, a third sample was obtained at six hours after pain onset, so that all patients had at least one sample six or more hours after pain onset. Mean arrival time was five hours after chest pain onset (< 2 hours in 3%); pain for more than 12 hours was an exclusion.
Serial creatine kinase MB actively documented acute myocardial infarction (MI) in 21 of the 773 patients (6%), 315 had unstable angina, 121 had stable angina, 12 had pulmonary embolism, 15 had heart failure, five had myocarditis, and 258 had no evidence of ischemic heart disease. All patients with acute MI and unstable angina were admitted. In the acute MI patients, troponin T was positive in 94% and I in 100%; in the unstable angina patients, T was positive in 22% and I in 36%. Among the patients with at least one positive troponin test, only in 58% of those with a positive T and 64% with a positive I was the test positive on arrival.
Among those with acute MI, 51% had a positive troponin T on arrival, 66% a positive I, and 53% a positive creatine kinase MB. At six hours, T was positive in 94%, I was positive in 100%, and CK-MB was positive in 91%. CK-MB was only elevated in 5% of the unstable angina patients on any test. Troponins were elevated in some of the patients with other cardiac diagnoses and pulmonary embolism, but only one patient without disease had a positive troponin I; seven patients with renal failure had positive troponin T, but not I, tests. CK-MB was elevated in 27 patients with negative troponins and no evidence of cardiac disease.
Follow-up at 30 days revealed 34 cardiac events and 20 deaths. If all troponin T tests were negative, the event rate was 1.1%, and if all troponin I tests were negative, the event rate was 0.3%. When the ECG was considered, no patients with a normal ECG and a normal troponin I test had an event. Troponins were powerful predictors of subsequent events (odds ratio 61 for I, 26 for T) vs. CK-MB (3.5), ECG ST segment depression (2.9), or ECG T-wave inversion (0.4). Hamm et al conclude that the new rapid troponin tests, especially troponin I, are highly sensitive for detecting myocardial injury in acute coronary syndromes, and negative tests are associated with low enough risk to permit safe discharge from the emergency room after an episode of chest pain.
COMMENT BY MICHAEL H. CRAWFORD, MD
The troponin tests are here, and most hospitals are now using them. This study extends previous study results in two ways. First, the new rapid qualitative tests seem to provide results as good as the more laborious quantitative tests. Second, the concept of using troponins to triage in the emergency department setting seems valid. Patients with positive tests clearly had more events and deserved further evaluation—usually as in-patients. Patients with negative tests had a low risk of events at 30 days. The data suggest that two negative tests or one at six or more hours after pain onset identifies a very low-risk group that can safely be discharged. Many of these patients may need further evaluation, but it can be done in the out-patient setting.The results suggest that troponin I is superior to T; sensitivity was generally higher, and there were less false-positives—especially due to renal insufficiency. However, since all patients with positive troponin T tests were admitted, the sensitivity of T may have been blunted vs. I. Both tests were superior to CK-MB in the time frame studies (6 hours), but older studies have shown that after 12 hours, CK-MB is 99% plus sensitive for acute MI and is still the gold standard for diagnosis. Also, CK-MB does not have the false-positive rate in unstable angina that the troponins have (one-third positive). Thus, the real value of the troponins is their ability to rapidly triage patients by six hours after pain onset to those who need to be admitted (positive tests) and those who could go home (negative tests).
Despite these impressive results, Hamm et al point out that the troponins are not a substitute for the history, physical examination, and ECG. The ECG is especially valuable for the immediate triage of the acute MI patient (ST segment elevation) to chemical or mechanical revascularization. Patients without obvious acute MI do not benefit from thrombolysis, but these patients need careful evaluation to exclude other life threatening illnesses such as aortic dissection, etc. Also, the true cost savings of a six-hour emergency room triage protocol using troponins has not been established.
Observation for another six hours with serial CK-MBs may be just as cost-effective and certainly is of equivalent accuracy. In addition, other tests may be required such as exercise ECG to safely discharge the patient. However, the rapid troponin assays are clearly a significant advance, and I agree with Hamm et al that every emergency room should have access to them.
At six hours after chest pain onset, which test most accurately detects acute MI?
a. Troponin T
b. Troponin I
c. CK-MB
d. LDH isoenzymes
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