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

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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.

Introduction

Incidence studies of dementia often rely entirely on follow-up assessments to identify cases, and subjects who do not complete a follow-up assessment are often omitted from the analysis. These subjects, however, cannot be assumed to be missing at random 1, 2 because dementia decreases life expectancy (3), and subjects who develop dementia may thus be more likely to have been too sick to participate or to have died before the follow-up examination. Selective attrition due to excess mortality and morbidity has long been recognized (4). A recent systematic review of longitudinal studies in older adults showed that older age and cognitive impairment—key factors in studies of dementia—were the primary and most consistent predictors of attrition (5). Although longitudinal studies of dementia routinely employ strategies to minimize attrition, such as shorter intervals between assessments or data collection from alternative sources, loss of subjects due to death is inevitable and can be substantial (e.g., from 7.0% 6, 7 to 22.4% (8)). Subjects are also lost to follow up because they refuse to participate after their baseline assessment, cannot be located, have moved out of the study area, or are otherwise unable to complete the study. It is unclear how the incidence of dementia among these subjects would compare to subjects who participate in a follow-up assessment, although this type of attrition has been shown to result in underestimates of cognitive decline (9).

Excluding subjects lost to follow up may affect the accuracy of dementia incidence rates and, in turn, hinder the utility of these rates to provide valid projections of future service needs. To assess the impact of attrition, information on dementia status is required for all subjects regardless of follow-up status, and a source of information common to all study subjects, such as health care utilization records, is preferable.

The objective of this study was to estimate the incidence of dementia and the influence of subject attrition. Assessment of incidence rates based on follow-up examinations and those based on health care utilization records (which are not affected by subject attrition) allowed estimation of the impact of loss to follow up and adjustment of incidence rates for attrition.

Section snippets

Study Population

The Manitoba Study of Health and Aging (MSHA) is a population-based longitudinal study of aging and dementia in community-based and institutionalized adults 65 years of age or older. The MSHA study design (Fig. 1) has been described elsewhere 10, 11. Briefly, in 1991, the population of Manitoba aged 65 years and older was 147,372, of whom 139,579 lived in the community and 7793 in institutions (nursing homes). Baseline data were collected on 1763 community and 189 institutional subjects in

Incidence of Dementia Based on Follow-up Assessments

Approximately 5 years after baseline, 117 subjects were diagnosed with dementia at follow-up clinical assessment among community subjects. Age-specific incidence rose from 4.3/1000 person-years to 79.6/1000 person-years from the age groups 65–69 years to 90 years and older, respectively (Table 1). The age- and gender-standardized incidence of dementia was 15.6/1000 person-years (95% confidence interval [CI: 12.4–18.7]). There was no significant difference in incidence of dementia between men

Discussion

Incidence rates for dementia and AD were calculated for a population-based sample 65 years or older from both the community and institutions. We considered follow-up examination and clinical assessment as the gold standard method to identify cases of dementia. Incidence estimates from this method, however, are potentially biased because completers may not be representative of the total population at risk. A unique feature of our study was the availability of health care utilization records,

Conclusions

We have shown that censoring of decedents biases incidence rates, resulting in underestimates of dementia incidence in our study. Loss to follow up of subjects who refused to participate or who were unavailable at follow up also produced underestimates of dementia incidence, but attrition of these subjects had less impact than did attrition of decedents. Sources of data available for all subjects, such as health care utilization records, can be used to correct incidence estimates for attrition

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    The Manitoba Study of Health and Aging (MSHA) was funded primarily by Manitoba Health (1990–1993) and Manitoba Health's Healthy Communities Development Fund (1995–1999). Additional funding was provided through the Canadian Study of Health and Aging (CSHA) by the Seniors Independence Research Program of the National Health Research and Development Program of Health Canada (Project No. 6606-3954-MC[S]). The results and conclusions are those of the authors and no official endorsement by Manitoba Health is intended or should be inferred.

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