The effect of definitions of activities of daily living on estimates of changing ability among older people.

Physical functioning status is often assessed using scales of activities of daily living and instrumental activities of daily living. Different ways of defining change in adequacy of performance of both activities of daily living and instrumental activities of daily living tasks may be used, including increasing difficulty in performance, the need for assistance and inability to do tasks. In this prospective study, we investigated the effect of different definitions of decline on estimates of physical function, and explored the relationships between decline in each activity of daily living individually and potential predictors. The study was based on a sample of 999 individuals aged 65 years or more who participated in a national survey of quality of life, of whom 531 (68% of those eligible for follow up) responded 12-18 months later. Different definitions of decline were used and the prevalence of decline was, depending on the individual activities of daily living item, used as an outcome in logistic regression models. The results showed that the strength of association with chronic diseases, demographic, psychological and environmental factors varied by altering the activities of daily living item used. Decline in ability to walk 400 yards was strongly associated with respiratory problems (odds ratio 3.5 [95% confidence interval 1.3-9.0]) while decline in ability to get on a bus was associated with musculoskeletal problems (odds ratio 2.8 [95% confidence interval 1.4-5.6]). In conclusion, the prevalence of decline varies by definition, and summary measures which are customarily used to describe disability, may be inadequate for the assessment and identification of predictors of decline in functional ability.