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

Objective
Both prevalent and incidentcohorts 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 incidentcohorts of people with a first acute myocardialinfarction (AMI) were formed from the samepopulation. They were analyzed independently in terms of baseline characteristics and survival to revascularization, another AMI, or death.

Results
55.7% of the prevalentcohort members were males, with a mean age of 71.0 years (standard deviation [SD]: 12.0). 59.0% of the incidentcohort 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 incidentcohort, mean time to AMI was shorter. Fewer prevalent cases underwent coronary artery bypass grafting or percutaneous transluminal coronary angioplasty.

Conclusion
Considerabledifferences existed between the two cohorts in terms of baseline characteristics and prognosis. Incidentcohortsderived from whole populations should be sought for estimation of survival.