Troponin-I in Acute MI: No Substitute for Clinical Judgment
ABSTRACT & COMMENTARY
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.
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.