Only 10 to 15 percent of patients who present with acute chest pain have a myocardial infarction (MI). Symptoms cannot be used to differentiate those who have had an MI from those who have not. Further, the electrocardiogram is diagnostic for myocardial ischemia in only 40 percent of patients. Cardiac enzyme assays have formed the basis for making this critical determination. Unfortunately, the standard creatine kinase-MB (CK-MB) assay is not reliable in excluding infarction until 10 to 12 hours after the onset of chest pain. Earlier diagnosis would have very specific advantages with regard to fibrinolytic therapy and triage. Studies have been undertaken to assess the utility of newer markers, such as troponin I, troponin T, creatine kinase-MB “subforms” and myoglobin. Zimmerman and associates present the results of the Diagnostic Marker Cooperative Study (DMCT), which was used to assess each of these enzyme assays.
The DMCT was a prospective, multicenter, double-blind study of 955 consecutively enrolled patients who presented to an emergency department with chest pain. Sensitivity and specificity were determined for conventional CK-MB, CK-MB “subforms,” myoglobin, troponin T and troponin I.
Patients underwent each test at hours 2, 4, 6, 10, 14, 18 and 22. The results are shown in the accompanying table
. The most reliable marker for early diagnosis (within six hours of onset of symptoms) was the CK-MB subform, with a sensitivity of 91.5 percent. This was followed by myoglobin, conventional CK-MB, troponin T and troponin I. The most reliable marker for late diagnosis (at 22 hours) was the conventional CK-MB, with a sensitivity of 95.7 percent. This was followed by troponin I, troponin T, CK-MB subforms and myoglobin.
Based on the results of this study, physicans can better discern the reliability of tests in ruling out myocardial ischemia. The authors found that CK-MB subforms were the most reliable marker for myocardial damage in the first six hours of symptom onset, a conclusion reached by Puleo and colleagues in an earlier article (Puleo PR, et al. Use of a rapid assay of subforms of creatine kinase MB to diagnose or rule out acute myocardial infarction. N Engl J Med 1994;331:561–6).