In this longitudinal study, which included multiple assessments of depressive symptoms and monthly assessments of disability over the course of 9 years, we found that the likelihood of experiencing mild disability increased with successively higher levels of depressive symptoms, whereas the likelihood of experiencing severe disability was elevated only among persons with high depressive symptoms. Furthermore, although depressive symptoms were associated with subsequent disability burden in both men and women, we found that level of depressive symptoms had a differential effect on men and women. Among men, moderate depressive symptoms were associated with mild disability, whereas both moderate and high depressive symptoms were associated with severe disability. Among women, the likelihood of experiencing mild disability increased as the level of depressive symptoms increased, whereas only high depressive symptoms were associated with severe disability.
Among community-living older persons, the association between subsyndromal depression and disability is well established (3
). In the current study, we confirmed this association using an CES-D score of 20 or higher to denote subsyndromal depression, as has been done in several prior studies (25
). Yet, relatively little is known about the association between depressive symptoms that fall below the symptom threshold for subsyndromal depression and the development of disability, despite the high prevalence of such depressive symptoms among older persons (9
). An earlier community-based study found that depressive symptoms not meeting the criteria for subsyndromal depression, but assessed at only a single point in time, were associated with the development of disability, assessed annually over the course of 6 years (4
). These findings, coupled with ours, provide strong evidence that a broad spectrum of depressive symptoms, including those below the threshold for subsyndromal depression, is associated with poor functional outcomes. Hence, recognizing and treating depressive symptoms that are below the threshold for subsyndromal depression has the potential to reduce the subsequent burden of disability among older persons.
The public health impact of disability is substantial, with several studies indicating that the utilization and cost of both formal and informal health care resources increase with worsening disability (38
). Nonetheless, prior studies of depressive symptoms have not attempted to distinguish between different types of disability severity, despite increasing evidence that the burden of disability varies widely among older persons (3
). We found that moderate depressive symptoms were associated with an increased likelihood of developing mild, but not severe, disability, whereas high depressive symptoms were associated with an increased likelihood of developing both mild and severe disability. These findings suggest that the failure to identify older persons with moderate depressive symptoms may represent a missed opportunity to prevent milder forms of disability through proper management of depressive symptoms. Furthermore, failure to recognize mild and severe disability may obscure the differential association between varying levels of depressive symptoms and this important health outcome.
The results of our study provide additional evidence that the association between depressive symptoms and disability is observed in both older men and women (3
) and suggest that there may be a sex difference, albeit modest, in the relationship between varying levels of depressive symptoms and the severity of subsequent disability. Specifically, although a broad spectrum of depressive symptoms was associated with severe disability among men, only high depressive symptoms were associated with severe disability among women. It is possible that sex differences in the reporting of depressive symptoms, with men being less apt to report such symptoms (41
), may have differentially affected the sex-specific rates of both moderate and severe depressive symptoms in the present study. Whether these differences could have led to the observed difference in the relationship between varying levels of depressive symptoms and the severity of subsequent disability is uncertain but should be the focus of future research.
There are several potential limitations to this study. First, the CES-D does not provide diagnostic criteria for the assessment of clinical depression. Nonetheless, the symptom scale provides useful information for identifying a range of symptom severity within individuals who otherwise would be classified as nondepressed. Because the CES-D includes somatic items (e.g., I could not get “going;” I did not feel like eating, my appetite was poor), it is possible that symptoms of physical illness such as fatigue or weight loss could have been attributed to depression, thereby inflating the CES-D score for some participants (42
). To address this possibility, we used a cut-point of 20 or higher on the CES-D, which previously has been recommended to minimize the likelihood of incorrectly categorizing older persons as having severe depressive symptoms (43
). Second, although we adjusted for the use of antidepressant medications, information regarding the dose, dosing schedule, adherence, indication, and start of treatment was not available. Because sex differences in the receipt and response to pharmacological and nonpharmacological treatment for depressive symptoms have been reported (44
), future research should evaluate whether sex differences in depression treatment may help to explain the sex differences found in the present study. Third, the current study did not evaluate potential mechanisms underlying the relationship between level of depressive symptoms and disability burden. It is possible, for example, that increasing levels of depressive symptoms could lead to reduced levels of physical activity (45
), which could subsequently influence disability burden in this population. We found that increased levels of physical activity were associated with a reduced likelihood of experiencing both mild and severe disability. Whether physical activity fully or partially mediates the association between level of depressive symptoms and disability burden in older persons should be the focus of future research. Fourth, we did not account for a prior history of disability in our analysis. This information was not collected during the baseline assessment. In addition, because we excluded 18-month intervals when disability was present during the relevant comprehensive assessment, a prior history of disability would not be available for another 9% of the intervals. Lastly, because our study participants included members of a single health plan, the generalizability of our findings to other older adult populations may be questioned. As previously noted, however (14
), the demographic characteristics of our study population, including years of education, closely mirror those of persons aged 70 years or older in New Haven County, which, in turn, are comparable to those in the United States as a whole, with the exception of race. New Haven County has a larger proportion of non-Hispanic whites in this age group than in the United States, 91% versus 84% (46
). Furthermore, generalizability depends not only on the characteristics of the study population but also on its stability over time (47
). The high participation rate, completeness of data collection, and low rate of attrition for reasons other than death all enhance the generalizability of our findings (47
) and at least partially offset the absence of a population-based sample.
Our findings indicate that depressive symptoms among older persons contribute substantially to the burden of disability over time and demonstrate the potential adverse consequences of depressive symptoms that do not reach the threshold for subsyndromal depression. Furthermore, our findings suggest that subthreshold depressive symptoms may be particularly problematic for older men, thereby emphasizing the need to take sex into account when evaluating the relationship between depressive symptoms and severity of disability. More broadly, our findings underscore the complexity of the relationship between depressive symptoms and disability. Identifying and managing a broad spectrum of depressive symptoms in older persons may ultimately help to reduce the burden of disability in this population.