a1 Department of Psychiatry and Behavioral Sciences, Center for the Study of Aging and Human Development, Duke University Medical Center, Durham, NC, USA
a2 Department of Biostatistics and Bioinformatics, Center for the Study of Aging and Human Development, Duke University Medical Center, Durham, NC, USA
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.
(Received May 30 2008)
(Revised February 19 2009)
(Accepted February 21 2009)
(Online publication April 09 2009)
Preliminary results of this study were presented in a poster session at the 132nd Annual Meeting of the American Public Health Association, Washington, DC, in November 2004.