A changing relationship between disability and survival in the elderly population: differences by age

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Abstract

Background

Longitudinal studies estimating the association between disability and mortality in the elderly population have typically assumed disability constant through the follow-up study period. Current knowledge indicates that such assumptions may not be appropriate. Our purpose was to examine this association (disability and mortality) taking into account the transitions in the disablement process.

Methods

1,294 participants (aged 65 and over) in the Health Interview Survey of Barcelona were followed up for a median of 8 years. Nine basic activities of daily living (ADLs) were measured at baseline and at the end of follow-up. Individuals were defined as “dependent” if they reported not being able to perform one or more of the activities without assistance. Survival analysis with delayed entry, age as the time variable, and disability as a time-dependent variable was performed.

Results

The rates of disability had increased by the end of the follow-up (from 42.0 to 60.0% among women and from 30.0 to 48.0%, among men); 7.5% of disabled women at baseline and 28.5% of men recovered from disability. The adjusted relative risk of dying for those with basic ADLs dependency varied with age: at 80 years of age it was 3.5 for women and 1.8 for men, while at 90 years it was 1.9 and 1.2, respectively.

Conclusions

Disability increases monotonically over time while the risk of mortality associated with disability varies with gender and age. Elderly disabled women should be considered a target group for intervention because they show higher rates of disability and are less likely to recover from disability. Our results illustrate the need to consider disability status as a time-dependent variable, to avoid an underestimation of its association with mortality.

Introduction

Disability is an important indicator of health status of the elderly people that has been shown to be strongly associated with mortality [1], [2], [3], [4], [5]. The importance of disability measured by limitations in performing certain activities of daily living (ADLs) relies on the fact that it provides a measure of the burden caused by suboptimal health. It is the result of a large number of factors such as underlying diseases, coexisting chronic conditions, psychologic aspects, and social and environmental factors [6]. ADLs have been extensively used for assessing the need for health services [7] and as eligibility criteria for long-term care [6], [8].

The relationship between disability and mortality has often been analyzed by means of logistic regression, with the main outcome being vital status. That is, the association has been assessed ignoring possible changes in the risk of death over time. Some studies overcame this limitation by taking into account the time elapsed from entry into the study until death or emigration or the end of the study using survival techniques. However, as far as we know, these studies have considered disability as a time-fixed variable, not taking changes into account during the follow-up period.

Wallace and Colsher [9] have pointed out that when considering disability and chronic illnesses, the change over time of exposures and their differential impact must be considered. This suggestion is consistent with previous studies that have shown that disability follows a dynamic pattern, with an increasing prevalence of dependence with age, but also with a small subset of individuals who are able to regain independence [10], [11]. As a consequence, the association between mortality and disability found with a long follow-up period and a fixed measure of disability may be misleading. Typically, the estimated association will be too weak. Moreover, there is little information about the changes in disability that elderly people experience over time and the effect of such changes on survival.

The objective of the present study was to describe the relationship of disability with mortality at different ages, allowing for changes in disability status during the follow-up period.

Section snippets

Sample

Data come from the Health Interview Survey of Barcelona, a longitudinal study of the noninstitutionalized elderly population, aged 65 and over, and resident in Barcelona (Mediterranean city with about 1,500,000 inhabitants [12] located in the north-east of Spain). The study was conducted to examine the relationship between health-related characteristics, the use and need of health services, and subsequent mortality. The used was derived from an adaptation of the supplement on aging in the 1984

Results

The distribution of the main variables of the study cohort at baseline is shown in Table 1. More than 60% of the subjects were women (n = 794); the mean age at entry into the study was 74.4 (SD = 7.0) for women and 73.5 (SD = 5.9) for men. The majority of men were married (80.2%), and were living with others (91.4%). Women were less educated (18.1% with no studies completed), mainly widowed (49.5%) or married (38.7%), and about a quarter of them lived alone. Only 5.6% of the men had overweight or

Discussion

The purpose of the present study was to examine the association between disability and mortality in the elderly population taking into account the possible changes in the disability status. We wanted also to evaluate the intensity of functional loss and its impact on mortality at different ages.

Compared to predominant survival analysis strategies for the elderly population, which define the survival time as the elapsed time from entry into the study until death, our approach via survival

Acknowledgements

This work was supported by a grant from the Fondo de Investigación Sanitaria (FIS Expte. No. 91/0629). Additional support was received from the Generalitat de Catalunya (CIRIT/1999 SGR 00240). Rosa Lamarca was supported by a grant from the Instituto de Salud Carlos III (97/4364). An abstract was presented at the 52nd Annual Scientific Meeting of the Gerontological Society of America (San Francisco, 1999).

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