Considerable differences exist between prevalent and incident myocardial infarction cohorts derived from the same population

Objective: Both prevalent and incident cohorts have been used in epidemiological and prognostic studies of ischemic heart disease (IHD). This study considers the differences between the cohort types. Study Design and Setting: Using linked primary care, secondary care, and death certification data, prevalent and incident cohorts of people with a first acute myocardial infarction (AMI) were formed from the same population. They were analyzed independently in terms of baseline characteristics and survival to revascularization, another AMI, or death. Results: 55.7% of the prevalent cohort members were males, with a mean age of 71.0 years (standard deviation [SD]: 12.0). 59.0% of the incident cohort members were males, with a mean age of 64.7 years (SD: 13.3). Over 5 years, a greater proportion of prevalent cases died from any cause (31.4% [95% confidence interval(CI): 28.6-34.3]) and IHD (18.5% [95% CI: 16.2-21.0]) than incident cases (18.0% [95% CI: 15.0-21.4] and 12.2% [95% CI: 9.7-15.2], respectively). Mean time to death was shorter in prevalent cases. There was a small difference in the numbers of subsequent AMIs between cohorts. In the incident cohort, mean time to AMI was shorter. Fewer prevalent cases underwent coronary artery bypass grafting or percutaneous transluminal coronary angioplasty. Conclusion: Considerable differences existed between the two cohorts in terms of baseline characteristics and prognosis. Incident cohorts derived from whole populations should be sought for estimation of survival.