Assays for the cardiac troponins, markers of acute myocardial infarction (AMI), have recently been improved and are becoming routinely available. Brogan et al compared the diagnostic accuracy of cardiac troponin I (cTn-I) to that of the conventional assay, creatine kinase-MB (CK-MB) in ED patients with and without AMI. Thirty-five patients diagnosed with AMI were compared to 136 patients diagnosed as not having sustained an AMI using standard criteria of symptoms, ECG abnormalities, and CK-MB level.
The authors found that cTn-I offered no advantage over CK-MB in sensitivity or specificity shortly following onset of symptoms. Within six hours of symptom onset, each test had 40% sensitivity and 98% specificity for the diagnosis of AMI. Among patients presenting at least 24 hours after symptom onset, however, cTn-I was much more sensitive than CK-MB (100% vs 57%, respectively). The authors also found that elevation of either marker was associated with an increased risk of adverse outcomes. (Brogan GX, et al. Evaluation of a new assay for cardiac troponin I vs creatine kinase-MB for the diagnosis of acute myocardial infarction. Acad Emerg Med 1997;4:6-12.)
COMMENT BY DAVID J. KARRAS, MD
Many emergency physicians yearn for a test to free them from the difficult process of deciding which of the many chest-pain patients are having an AMI. The cardiac troponins will not fit this bill. This study is consistent with prior research finding that cTn-I remains elevated much longer than CK-MB and thus is a better marker of recent infarction, but that neither is useful as a marker of AMI among patients with chest pain of less than 6-12 hours in duration.1,2
It is important to note that almost all of the participants in this study were judged by their treating physicians to merit hospitalization; i.e., this is not a study of "all comers" with chest pain. Furthermore, the ultimate diagnosis of AMI was based, in part, on the results of one of the markers (CK-MB) that the study attempted to investigate. Any bias from this, however, would likely favor CK-MB, and no such advantage was found. It appears that there is little role for cTn-I for the ED diagnosis of AMI.
References
1. Adams JE, et al. Cardiac troponin-I: A marker with high specificity for cardiac injury. Circulation 1993;88:101-106.
2. Tucker JF, et al. Early diagnostic efficiency of cardiac troponin I and troponin T for acute myocardial infarction. Acad Emerg Med 1997;4:13-21.
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