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A Prospective Cohort Study

Walter A. Kukull, PhD; Roger Higdon, PhD; James D. Bowen, MD; Wayne C. McCormick, MD; Linda Teri, PhD; Gerard D. Schellenberg, PhD; Gerald van Belle, PhD; Lance Jolley, BS; Eric B. Larson, MD

Arch Neurol. 2002;59:1737-1746.

Context  Age-specific incidence rates for dementia and Alzheimer disease (AD) are important for research and clinical practice. Incidence estimates for the United States are few and vary with the population sampled and study design; we present data that will contribute to a consensus of these rates.

Objectives  To provide age-specific incidence estimates for dementia and AD and to estimate the association of sex, educational level, and apolipoprotein E genotype with onset.

Design  Prospective cohort study; begun in 1994 with follow-up interviews every 2 years.

Setting  Members of community-based, large health maintenance organization with demographics consistent with the surrounding base population; diagnostic evaluation by university-based study clinicians.

Subjects  Random sample of subjects aged 65 years or older from the base population; dementia free, nonnursing home residents. Of 5422 who were eligible, 2581 were enrolled, and 2356 had at least 1 follow-up evaluation (10 591 person-years of observation).

Main Outcome Measure  Dementia and Alzheimer disease diagnoses were based on standard criteria. Age-specific incidence rates were calculated using a person-years approach with Poisson distribution confidence intervals. Cox proportional hazards model analysis was used to examine other factors.

Results  Two hundred fifteen cases of dementia and 151 cases of AD were diagnosed. Incidence rates for dementia and AD increase across the 5-year age groups; AD rates rise from 2.8 per 1000 person-years (age group, 65-69 years) to 56.1 per 1000 person-years in the older than 90-year age group. The rates nearly triple from the 75-to-79-year and 80-to-84-year age groups, but the relative increase is much less thereafter. Sex was not associated with AD onset. Educational level (>15 years vs <12 years) was associated with a decreased risk of AD; however, the association was also dependent on the baseline cognitive screening test score.

Conclusions  Our dementia and AD incidence rates are consistent with recent US and European cohort studies, providing clinicians and researchers new information concerning the reproducibility of incidence estimates across settings. Increased risk was associated with age and the apolipoprotein E genotype; also with a low baseline cognitive screening test score. Educational level was inversely associated with the risk of dementia and positively associated with the baseline cognitive test score; thus, detection of AD by the screening test could also be influenced by educational level.

From the Departments of Epidemiology (Drs Kukull and Higdon), Neurology (Drs Bowen and Schellenberg), Medicine (Drs McCormick and Larson), Psychosocial and Community Health (Dr Teri), Environmental Health (Dr van Belle), and Biostatistics (Dr van Belle and Mr Jolley), University of Washington, Seattle, and the Department of Geriatric Research and Education Clinical Center, Seattle Veterans Administration Puget Sound Health Care System (Dr Schellenberg).

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