Assessing the requirement for the 6-hour interval between specimens in the American Heart Association classification of myocardial infarction in epidemiology and clinical research studies

Background: The American Heart Association (AHA) case definition for acute myocardial infarction (AMI) requires an "adequate set" of biomarkers: 2 measurements of the same marker at least 6 h apart. A sensitive troponin assay might detect significant changes in concentration earlier. We determined AMI prevalence, using protocols with shorter intervals between measurements, with and without incorporating the time from onset of symptoms. Methods: The AHA case definition was used to retrospectively assign a diagnosis in 258 patients presenting to the emergency department with symptoms of cardiac ischemia. AMI was diagnosed if either specimen in an adequate set had a cardiac troponin I (cTnI) above the 99th percentile (AccuTnI® >0.04 μg/L; Beckman Coulter) with a ≥20% change in concentration between specimens. We assessed positivity for AMI after progressively decreasing the time interval between specimens in specimen sets. In addition, for each patient, 2 additional specimen pairs were selected: pairs collected at least 1 h apart with 1 specimen being either 2:3 h after onset or ≥6 h after onset. Results: When we used the AHA definition, the AMI prevalence was 35.7%. Prevalence was not significantly diminished when the interval between specimens was ≥5, ≥4, or ≥3 h (36.4%, 34.5%, and 33.7%, respectively) compared with the AHA ≥6 h interval. When the time from onset of symptoms was included in the specimen selection algorithm, a 1-h interval was sufficient provided that at least one specimen was collected ≥6 h after onset (prevalence, 34.1%; P = 0.48 vs AHA definition). Conclusion: A sensitive cTnI assay in specimen sets with time intervals 23 h, or having one specimen ≥6 h after onset, gave an AMI prevalence equivalent to the AHA definition.