Should creatine kinase-MB index be eliminated in patients with indeterminate troponins in the ED?

The objective of this study is to determine whether creatine kinase-MB (CK-MB) index (CK-MBi) is useful in the evaluation of acute myocardial infarction (AMI) in patients with indeterminate troponin (Tn) in the emergency department (ED).

A retrospective cohort study was conducted of patients at an urban academic ED with over 55 000 annual visits who underwent Tn T (Roche, Indianapolis, IN) and CK-MB testing. One year of ED patients who had Tn testing were identified, and their corresponding CK-MBi was examined to find patients with indeterminate Tn (0.01-0.09) and positive CK-MBi (>6.0). Subsequent cardiac enzymes and hospital course were reviewed to identify patients diagnosed with AMI. A 95% confidence interval around point estimates were used in data analysis.

Over 1 year, 11 718 initial Tn were identified. Indeterminate Tn was seen in 2512 cases. Of these, 28 had positive CK-MBi. Of the 28, 5 were judged by treating physicians to be having AMI and underwent cardiac catheterization. Of the 5 patients, 4 had subsequent positive Tns on serial enzyme testing. One of the patients thought to be having AMI had no coronary artery disease on catheterization. The rate of true positive CK-MBi with indeterminate Tn was 0.16% (95% confidence interval, 0.04%-0.41%).

Initial results identify rare cases of AMI where CK-MBi is positive in the setting of indeterminate Tn. However, most patients with indeterminate Tn and positive CK-MBi were not judged to be having AMI. In most cases, CK-MBi is not positive with indeterminate Tn and when positive more commonly confuses the picture. This suggests CK-MBi could be eliminated in patients with indeterminate Tns.