Background Functional disability and depression might be related to an increased risk of future dementia. We aimed to examine the occurrence of functional disability and depression over an eight-year period and evaluate the predictive role of these changes in dementia incidence.

Methods The data used for these analyses are from 1670 men and women aged 65 and older from the English Longitudinal Study of Ageing (ELSA), an on-going, open, prospective cohort study. Seven waves of data between 2002 and 2014 were analysed.

Dementia was determined by doctor-diagnosis combined with a score above the threshold of 3.38 on the Informant Questionnaire on Cognitive Decline in the Elderly.

Independent linear growth models with time-invariant covariates and a categorical distal outcome (dementia incidence in 2014) were used to examine whether different trajectories of functional abilities (activities of daily living, ADL; and instrumental activities of daily living, IADL) or depressive symptoms (Centre for Epidemiologic Studies Depression Scale, CES-D) between 2002 and 2010 were predictive of new dementia cases four years later.

Results We identified three independent patterns of functional and depressive symptoms trajectory. Most people showed no functional impairments over time (‘class III’) in either ADL (86%) or IADL (90%), a small group showed a steep deterioration (‘class II’) in ADL (5%) and IADL (3%), while the others show a substantial disability in either ADL (9%)or IADL (7%) at baseline with only a slow improvement over time (‘class I’). Similarly, most participants (79%) did not experience depressive symptoms during the study period (‘class III’), a small group (9%) showed increased depressive symptoms over time (‘class II’), while the others (2%) started with depressive symptoms and experienced a minor improvement over time (‘class I’).

After adjustment for age, sex, education and wealth, relative to participants in ‘class III’, study members who experienced a sharp deterioration in either measure of functional capabilities or depressive symptoms (‘class II’) were markedly more likely to be classified with dementia four years later (ADL: Odds Ratio (OR)=3.29 (95% Confidence Intervals (CI) 0.37–6.21); IADL: OR=14.59 (95% CI 2.09–26.09) and CES-D: OR=3.94 (95% CI 1.27–6.61)).

Conclusion Our results showed heterogeneity in all measures of functional abilities or depressive symptoms over time, suggesting that a steeper decline in each of these functions is not necessarily a normative process, but could constitute an early indication of neurodegeneration and pre-clinical symptomatology.