Original articles
Prevalence of disability in three birth cohorts at old age over time spans of 10 and 20 years

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Abstract

The prevalence of disability at the age of 75+ measured by the Katz Index of Activities of Daily Living (ADL) was compared among three birth cohorts: those born ⩽ 1903 (n = 348), those born ⩽ 1913 (n = 586), and those born ⩽ 1923 (n = 758). Significant risk factors for disability were female sex and age; the cohort effect was not significant. The prevalence rates of disability were 29.0% (95% CI 24.2–33.8), 34.8% (30.9–38.7), and 28.8% (25.5–32.0) for the first, second, and third cohorts. In the age group 75–79 years the rates were 20.1% (95% CI 13.8–26.4), 25.5% (20.2–30.7), and 14.4% (10.6–18.1). The change was due to the declining disability of women. The distributions in the three cohorts based on the numbers of ADL limitations did not differ. As far as the whole aged populations were concerned, longer life was not accompanied by improving health.

Introduction

The trends in the prevalence of disability among the increasing elderly populations constitute a fundamental issue in the planning and delivery of health and social services 1, 2, 3. Increasing life expectancy does not inevitably mean improving health at the population level. If longer life is a consequence of a declining incidence of diseases, it is obviously accompanied by improving health 3, 4. This alternative may at least delay the increase of the needs for care among an elderly population [5]. If, on the other hand, longer life is due to the alternative that sick persons are saved from death, the time spent with ill health increases, and the longer life is not accompanied by improving health 3, 4, 6. This alternative may increase the need for care.

Both alternatives have been supported among researchers. In the 1970s and the early 1980s, it was suggested that the period of disability occupied a larger part of the lengthening life span causing an increased need for care 7, 8, 9. On the other hand, Fries et al. suggested that the rectangularization of the survival curve shortened the period of disability decreasing the needs for care 10, 11.

In the National Long Term Care Survey (NLTCS), the age-specific prevalence rates for activities of daily living (ADL) impaired or institutionalized persons declined from 1982 to 1994 in the age group of 75–84 from 24.7% to 21.4%, and in the age group 85+ from 57.3% to 52.7% 5, 12. The community-based 70+ population in the National Health Interview Survey (NHIS) in 1984, 1986, 1988, and in 1990 showed somewhat lower rates. On the other hand, among the community-based 70+ population of the Longitudinal Study on Aging (LSOA) the prevalence of ADL disability among both sexes varied within 7.3–9.2% between 1984 and 1990 showing an increase in ADL disability in the earlier years and no change in the later years [13]. However, Crimmins et al. suggest that both the NHIS and the LSOA show some fluctuation rather than a clear trend in the prevalence of disability [13].

In the National Mortality Followback Survey the percentages of subjects with no disability in ADL increased significantly from 1986 to 1993 only among those aged 85 years and over. By sex, the change was from 20.4% to 30.0% among men and from 10.7% to 18.9% among women. There was no change among those aged 65–84 years [14]. Freedman et al., who used functional limitations in seeing, lifting and carrying, climbing, and walking as outcome measures for non-institutionalised Americans, found improvements in functioning among those aged 65–79 years and those aged 80 years and over from 1984 through 1993 in the Survey of Income and Program Participation [15].

Ford et al. compared two samples of non-institutionalized aged people in the City of Cleveland in 1975 and 1987 [16]. The prevalence for ADL disability decreased from 44.2% in 1975 to 43.0% (odds ratio 1.72) in 1987 in the age group of 77–88 years, and seemed to change from 76.9% to 81.8% (odds ratio n.s.) in the age group of 89+ years, respectively. In the cohorts aged 65–76 years, however, the prevalence of ADL disability increased from 17.1% to 17.6% (odds ratio 1.72) due to the increasing social burden of chronic illness and poverty.

In Melton Mowbray, Leicestershire, England, a comparison of two cohorts (belonging to general practice populations) taken in 1981 and 1988 at the age of 75+ years pointed out significant increases in independence in five activities of daily living, but no differences in four [1]. The British General Household Study (GHS) showed a marked drop at any specified old age in the proportions unable to perform four activities of ADL from 1976 through 1994 [17]. Among the age group of 75–79 years, the drop was from 18% to 6% for men and from 22% to 6% for women. Among those aged 80–84 years, the figures for men were 25% and 13% and those for women 36% and 15%. It was an interesting phenomenon that most of the change seemed to take place between 1976 and 1980. The figures even fluctuated from 1980 to 1994.

The objective of this article is to describe the prevalence of disability among three birth cohorts at the age of 75+ years in a geographically defined area. The previous measurements of disability among community-dwelling and institutionalized populations had been made in 1979 and 1989 [18]. In 1999 the third cohort was examined after a time span of 20 years.

Section snippets

Populations and methods

The study area covered two rural municipalities, Pyhäjärvi and Kärsämäki, in the district of Northern Ostrobothnia, Finland. The proportions of the population aged 75+ were 3.6%, 5.5%, and 7.8% in 1979, 1989, and 1999, respectively. The increase in the elderly population was due to the natural ageing of the permanent inhabitants. The decrease in the total population (Table 1) was due to outmigration of young working-aged population. Migration among the elderly people was insignificant.

Statistical analyses

The distributions of the cohorts in terms of ADL limitations were compared using the Kruskal-Wallis analysis of variance. The prevalence of disability among the cohorts was analyzed using logistic regression modelling, where age (continuous), sex and year of examination were taken into the model as possible risk factors. The results are presented using odds ratios (OR) supported by 95% confidence intervals (95% CI). The computation was carried out using SPSS/Win (Version 9.0) software [22].

Results

The 75+ population increased from 1979 to 1989 and further to 1999 in both sexes and in all age categories. The mean age in the first cohort was 79.6 years, in the second 79.8 years, and in the third 80.9 years. The mean age of the subjects lost from the population was 79.5 years in 1979, 80.6 years in 1989, and 79.6 years in 1999. The 1979 cohort included 57 (16%) persons in long-term institutional care, while the 1989 cohort included 82 (14%) such persons and the third cohort only 33 (4.4%).

Discussion

Chronic disability, especially in aged populations, is a sensitive measure of age-related changes in the health and biological fitness of aged people [5]. It was measured by the validated and widely used Katz Index of ADL. The inter-rater validity of the index has been found to be good 19, 21, as have the coefficients of reproducibility and scalability in evaluations made by one rater [23]. The fact that the same home nurse rated the populations in the same area in 1979, 1989, and 1999 by

Acknowledgements

We are greatly indebted to the public health nurse, Marketta Torola-Haapanen, for her skilful assistance in collecting this material in 1979, 1989 and in 1999. This research was supported by grants from the Foundation for Municipal Development and from the Hospital District of Northern Ostrobothnia.

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