Impact of the Look-Back Period on Identifying Recurrent Myocardial Infarctions in the Danish National Patient Registry

PURPOSE: Studies examining myocardial infarction (MI) often seek to include only incident MIs by excluding recurrent MIs. When based on historical data, identification of previous MI depends on the length of the look-back period. However, international registries often cover a short time period, consequently containing left-censored data, making it impossible to determine if a first MI in a period is truly an incident MI. We evaluated whether the proportion of MIs identified as recurrent MIs depends on the look-back period, and how including recurrent MIs in a planned incident MI cohort impacts survival estimates. PATIENTS AND METHODS: We used the Danish National Patient Registry, covering all Danish hospitals since 1977 to identify first MIs during 2010–2016 (index events). The hospital registry history preceding the index event was then searched for previous MIs. We plotted the proportion of index events identified as recurrent MIs as a function of the look-back period. Moreover, we calculated 5-year all-cause mortality and confidence intervals (CIs) using the 1-Kaplan–Meier method for five cohorts based on the index events and defined by look-back periods of 0, 5, 10, 20, and up to 39 years. RESULTS: Among 63,885 index events, 3.4% were identified as recurrent MIs with 5 years of look-back, 7.9% with 10 years, 14% with 24 years, and 15% with up to 39 years. All-cause mortality risk was 36% (95% CI: 36–37%) with 0 years of look-back, 35% (95% CI: 35–36%) with 5 years, 35% (95% CI: 35–36%) with 10 years, 34% (95% CI: 34–35%) with 20 years, and 34% (95% CI: 33–34%) with up to 39 years. CONCLUSION: Most recurrent MIs were identified with a look-back period of 24 years. Including recurrent MIs in a planned incident MI cohort, due to shorter look-back periods, overestimated the mortality risk.