The complex relationship between depressive symptoms and functional limitations in community-dwelling older adults: the impact of subthreshold depression.

BACKGROUND: Depressive symptoms above screening thresholds have been shown to predict functional decline in older adults. Less is known about the impact of subthreshold depression, and whether more symptoms confer significantly greater risk compared to fewer symptoms.
METHOD: Using data from the Duke Established Populations for Epidemiologic Studies of the Elderly (EPESE) collected over 10 years, we used repeated-measures mixed models to predict functional change by depression status at the prior (index) in-person interview. Depressive symptoms were measured using a modified version of the Center for Epidemiologic Studies Depression Scale (CES-D). Subthreshold depression was operationalized as 6-8 symptoms and CES-D-defined depression as 9-20 symptoms in the previous week. Three domains of functional status were assessed at the subsequent in-person interview: limitations in basic activities of daily living (ADL), instrumental ADL (IADL) and mobility.
RESULTS: Controlling for race, sex, age, education, marital status, cognitive status, health status, self-perceived health, perceived social support and functional status at the index interview, having 6 depressive symptoms predicted an increase of 0.12 IADL limitations 3-4 years later (p=0.03). The incremental effect of CES-D-defined depression (9 symptoms compared to 6-8 symptoms) was not significant, suggesting that the effect of more symptomatic depression did not add to that of subthreshold depression. CES-D score modeled as a continuous variable predicted functional change for all domains, but the relationship was not linear, supporting a possible threshold effect.
CONCLUSIONS: The relationship between depressive symptoms and functional change is complex, not necessarily linear, and may vary by tasks assessed.