Defining Myocardial Infarction

The diagnosis of acute myocardial infarction (AMI) as defined by the World Health Organization (WHO) was based for many years on the presence of two of three possible criteria: clinical symptoms compatible with AMI, typical electrocardiogram changes, and increases in markers of cardiac injury. However, because of the very good sensitivity and specificity of creatine kinase-MB (CK-MB), it eventually became rare to diagnose AMI in the absence of elevations of this biomarker. Thus, although never formally embraced by WHO, the clinical diagnosis of AMI became dependent on elevation of a biomarker of myocardial injury in the appropriate clinical setting. This approach evolved further with the development of cardiac troponin and its integration into the definition of MI by the European Society of Cardiology and the American College of Cardiology. The present diagnostic standard for MI is thus based on the following biomarker criteria: Maximal concentration of troponin T or I exceeding the decision limit (99th percentile of the values for a reference control group) manifesting a dynamic pattern on at least one occasion during the first 24 h after the index clinical event; if the value is between the 99th percentile and the 10% coefficient of variation level, caution is warranted because analytic false-positive results can occur. In the unusual situation in which troponin assays are not available, the value of CK-MB (preferably CK-MB mass) exceeding the 99th percentile of the value for a reference control group and manifesting a dynamic pattern can be used for diagnosis.