Effects of screening errors and differential mortality on the estimation of the incidence of dementia in the Canadian Study of Health and Aging

Estimating the Incidence of Dementia: The Impact of Adjusting for Subject Attrition Using Health Care Utilization Data

PURPOSE

To estimate incidence rates for dementia and the impact of subject attrition on these rates.

METHODS

Crude, age- and gender-specific incidence rates of dementia and Alzheimer's disease were calculated using person-years analysis and Cox proportional hazard models in a population-based cohort study of 1952 adults aged 65+ years in Manitoba, Canada. Rates were standardized to the nondemented population using the direct method. Ratios of incidence rates comparing completers to subjects who had died, refused, or were unavailable for follow up were based on health care utilization data (available for all subject groups) and used to adjust rates for attrition.

RESULTS

Decedents had a significantly higher incidence of dementia than did subjects who completed the follow-up assessment. The incidence in subjects who refused or were unavailable at follow up was intermediate between decedents and completers. Adjusted for attrition, the standardized dementia incidence rate for community and institutional subjects was 25.3/1000 person-years, significantly higher than that based on follow-up assessments only (17.8/1000 person-years; 95% confidence interval: 14.3-21.4).

CONCLUSIONS

The impact of loss to follow up on incidence rates varies depending on the reason for subject attrition. Incidence studies of dementia should develop strategies to characterize and address subject attrition to avoid underestimating disease incidence.

The incidence of dementia in Canada

Objective: To estimate the incidence of dementia, including AD, among Canadians aged 65 and over.

Methods: A 5-year cohort study of 10,263 seniors was undertaken, including community and institutional samples. The baseline study in 1991 identified 1,132 prevalent cases of dementia through screening and clinical examination. The remaining 9,131 cases formed the incidence study sample and were rescreened and selectively reexamined in 1996. Incident cases were diagnosed using established criteria. Incidence was estimated based on the 1991 population, and included data on those who died between the first and second phases of the study.

Results: Of the nondemented cohort who remained alive in 1996, 5,432 people in the community (88.3%) and 210 (91.3%) in the institutional sample participated in the incidence study. Nine hundred sixty incident cases were identified; the overall age-standardized incidence rates were 21.8 (women) and 19.1 (men) per 1,000 nondemented persons per year. This translates into 60,150 new cases of dementia per year in Canada. The logarithm of the rates rises linearly with age, but suggests a slight slowing of growth in incidence in the oldest age groups.

Conclusions: Our incidence estimates lie toward the upper end of the range of incidence estimates found in other studies. Nonetheless, we calculate that several factors may have biased our estimates downward, suggesting that the incidence of dementia may be higher than many studies have reported.