Evaluation of a New Assay for Cardiac Troponin I vs Creatine Kinase-MB for the Diagnosis of Acute Myocardial Infarction

Objective : To compare a new assay for cardiac troponin I (cTn-I) with an assay for creatine kinase-MB (CK-MB) for the diagnosis of acute myocardial infarction (AMI).
Methods : A prospective cross-sectional study of patients presenting with symptoms consistent with cardiac ischemia was performed at a university teaching hospital. Serum sampling for cTn-I and CK-MB was performed at 0, 1, 3, 8, and 16 hours after presentation. Normal values were defined as CK-MB ≤ 7 ng/mL and a relative index ≤ 2%, cTn-I ≤ 1.4 ng/mL. Final diagnosis was made using World Health Organization criteria, including standard enzyme sampling. Consecutive patients with AMI were compared with a randomly selected subset of patients without AMI to determine the sensitivity and specificity of the cTn-I and CK-MB assays for AMI, stratified by time from symptom onset. The ability of the biochemical cardiac markers obtained within 6 hours of symptom onset to predict later complications or need for interventions was assessed using odds ratios (ORs).
Results : Thirty-five patients who had AMI were compared with 136 patients who did not have AMI. The sensitivities and specificities of the cTn-I and CK-MB assays, stratified by time from symptom onset, were: Patients who had elevations in either CK-MB or cTn-I within 6 hours of symptom onset were at increased risk for cardiovascular complications and/or interventions (CK-MB, OR 5.8; cTn-I, OR 6.3).
Conclusion : cTn-I was as sensitive and specific for AMI as was CK-MB in ED patients who presented within 24 hours of symptom onset. However, cTn-I was more sensitive in patients who presented ≥ 24 hours after symptom onset. Elevations of either marker within 6 hours of symptom onset predict an increased risk of complications and/or need for interventions.