This study had two major findings. First, depressive symptoms were not associated with mortality in a cohort of 3,767 primary care patients. Second, among a representative subsample of 400 of these older adults, patients with depressive symptoms at baseline reported nearly twice the functional impairment of older adults without depressive symptoms, and these patients were more likely to report depressive symptoms and functional impairment at subsequent reinterviews. Also, worse physical functioning and a reported decline in physical functioning were independently correlated with an increase in depressive symptoms, even when accounting for patients' prior level of depressive symptoms.
This study builds on the results of previous reports by following a large primary care sample of older adults for up to 6 years, explicitly documenting the change in depressive symptoms and functional impairment over time, and examining risk of mortality while controlling for a host of confounding clinical variables known to increase risk of death.
In a 1992 review, Thomas et al. suggested that prior studies reporting such an association between depressive symptoms and mortality among older adults tended to enroll patients from specialty psychiatry settings who meet criteria-based definitions of depressive disorders.18
These studies often failed to control for important clinical covariates such as comorbid conditions or smoking history. Prior studies reporting no association between depression and mortality have tended to enroll community-based samples and measure depression severity using symptom-based scales. For example, in a study of 1,855 community-based older adults, Thomas et al. found no association between depressive symptoms at baseline and survival time over 3 years.18
Similarly, in a population-based study of 1,612 adults aged 50 years and older, Gallo et al. found no increase in the relative risk of 13-year mortality among patients with major depression or with depression syndrome with sadness.39
In contrast, in a population-based study of 3,560 people, those with major depressive disorder determined using the Diagnostic Interview Schedule had an increased relative risk of death in proportional hazards models adjusted for age only.40
The current study extends these prior reports by enrolling a large sample of primary care patients aged 60 years and older for whom we had access to several important clinical variables previously demonstrated to predict mortality in older adults.412–43
The patient sample enrolled in the current study is older and, because we enrolled subjects visiting primary care clinics, likely had a greater burden of medical illness. Thus, these patients have a higher expected mortality and multiple competing potential causes of death. Nonetheless, we found no association between depressive symptoms and mortality. The strongest predictors of mortality in this cohort were smoking history, greater age, male gender, and evidence of poor nutrition.
Although mortality rates did not differ between those with and those without depressive symptoms at baseline, patients with depressive symptoms did report significantly greater physical impairment. This impairment was reported for relatively high levels of functioning, such as communication and social interaction, as well as for more basic functioning such as body care. The disparity in functional impairment between those with and those without depressive symptoms persisted several years after the baseline interview.
The observation of excess physical disability among the patients with depressive symptoms presents a paradox. Because previous studies have demonstrated that functional impairment is associated with survival, one would have expected to find that patients with depressive symptoms had excess mortality at least in part due to their reported excess functional impairment.41–43
The SIP instructions specifically ask if it is because of the patients' health that they are unable to perform the activity.35
The fact that we did not find this association raises the question of whether these patients' depressive symptoms caused them to overestimate their physical impairment 44
—a possibility that merits future research.
One reason for overestimating physical impairments may be the patients' attempts to explain or express their impairment on the basis of physical rather than psychiatric illness. Alternatively, their physical impairment could be quite real, but because part of the etiology of this impairment is an affective disorder, the excess physical impairment may not confer the same excess mortality attributable to physical impairment due to chronic medical illness.
There are several limitations with these data. First, we measured symptoms of depression rather than criteria-based diagnoses. It is possible that patients with criteria-based major depressive disorder do have excess mortality due to their affective disorder; these data cannot address this issue. Also, it is conceivable that patients with depressive symptoms were more likely both to move out of state and to die, thereby causing misclassification bias. Second, among the reinterview sample, some patients have longer follow-up and more reinterviews and thus contribute more observations to the analysis using the generalized estimating equation method. However, we repeated our analyses limiting the data to only one observation per patient, and the results were unchanged. Third, our study of the relation of depressive symptoms to function is limited to 400 of the larger screened sample of 3,767 older adults. Finally, we did not determine whether worsening function preceded an increase in depressive symptoms, or vice versa.
Consistent with previous studies, the best predictor of future severity of depressive symptoms was prior level of depressive symptoms.8, 10, 13, 25, 45–52
However, declining functional status was also associated with worsening depressive symptoms. Thus, there were aspects of depressive symptoms and functional disability that were not associated with excess mortality. Future studies should seek to determine if successful treatment of late-life depression results not only in a reduction in depressive symptoms, but also improvement in physical function. This is especially important among the cohort of older adults treated in primary care settings because these patients typically suffer from multiple chronic medical illnesses that contribute to functional impairment. Finally, consideration must be given to the possibility that psychiatric and physical symptoms each simply occur together and contemporaneously to some degree in each patient.53
This possibility places a premium on treatment strategies and health systems capable of addressing the full range of patients' problems simultaneously.54, 55