At baseline, the sample ranged in age from 65 to 105 years, 54% were African American, 35% (both African Americans and Whites) were male. Over half had not gone beyond grade school. Health was rated between good and fair. A substantial proportion reported impairment in mobility and instrumental ADL, but few had any basic ADL impairments. On average, cognitive status was in the unimpaired range. At baseline, a majority had experienced one stressful life event (mean (standard deviation) 0.67 (0.95)); mean network size was 13.82 (7.10), and mean social interaction was 13.97 (6.76). The mean CES-D score was 3.2 (3.4), with 9.6% reporting 9 or more depressive symptoms. In the early years approximately 5% of the sample died each year, the percentage dying each year increasing with the age of the study.
A four class model best fit the data. Participants were assigned to the class to which they had the highest probability of belonging. The four trajectory classes were a stable class with few depressive symptoms (n = 3043; 76.6%); a stable class with many depressive symptoms (n=216; 5.4%), where “stable” indicates consistency in number of depressive symptoms (score on the CES-D) over time; a class with an increasing number of depressive symptoms (labeled “decliners”) (n=396; 10.0%); and a class with a decreasing number of depressive symptoms (“improvers”) (n=318; 8.0%) (). At baseline, the stable low symptoms class reported between 1 and 2 depressive symptoms, a level maintained throughout the study. The stable high symptoms class had a mean baseline modified CES-D score of 12, and maintained a score greater than 10 throughout. In the decliner class score increased from a baseline average of around 3 symptoms to approximately 9 symptoms (the cut-point that indicates clinically significant depressive symptoms on this version of the CES-D) 6 years later, before reversing slightly in the tenth year. Finally, the improver class initially had an average score of approximately 8.25, i.e., close to the CES-D cutpoint, improved over the first 6 years (to around 3 symptoms, the same starting point as the improver class), and then deteriorated slightly. Uncontrolled examination of the association of baseline characteristics with trajectory classes showed significant differences for all variables () across classes.
Trajectory classes of depressive symptomatology
Baseline Characteristics of the Trajectory Classes (mean (standard deviation), or N (%)) (n=3973)
The stable low symptomatology class was younger, included a larger proportion of men, had more education, the best health status on each of the health measures, the fewest stressors, the highest social support, and (as with the improver class), the lowest death rate over 12 years (60%). The stable high symptomatology class was more likely to be White, reported the poorest health status, the largest number of stressors, the poorest social support, and two thirds died within 12 years. Compared to the improvers, the decliners were over a year older, included a larger proportion of African Americans, men, and people with less education. Their self-rated health, functional status, stressful life events and social support was comparable to that of the stable high symptomatology class; they had the poorest cognitive status of all the classes. At 73%, they had the highest rate of death.
Controlled analyses (multinomial logistic regression) included demographic (age, race, sex, education), health status (self-rated health, functional status, cognitive status), and social factors (stressful life events, social network, social interaction) (). Overall, each of the demographic, health status, and social factors variables, with the anticipated exception of age, significantly discriminated the four trajectory classes of depressive symptomatology. The weakest, aside from age, was race.
Multinomial Logistic Regression. Baseline Associates of Trajectory Classes of Depressive Symptomatology (Reference trajectory class = stable low depressive symptomatology (76.6%))
Separate analysis of demographic and CES-D characteristics predictive of death indicated that odds of death increased by 8% for each additional year of age (Odds Ratio [OR] =1.08; 95% Confidence Interval [CI]1.07-1.09, p <.0001), was 56% higher for men (OR = 1.56; 95% CI 1.34-1.82, p <.0001), declined by 3% for each additional year of education (OR = 0.97; 95% CI 0.95-0.99, p = .0022), and increased by 3% for each additional depressive symptom (OR = 1.03; 95% CI 1.01-1.06, p = .0034). Race was not a significant predictor. The inclusion of CES-D score in the model barely modified the demographic characteristics, indicating that depressive symptomatology made a unique contribution.
For each category of variables, we first make comparison with the reference group (stable low depression symptoms), and then compare the remaining three trajectory classes, in particular to identify significant differences among them.
Compared to the stable low symptomatology class, women had increased odds of being in one of the other classes – by 45% for the decliner class, and about double the odds for being in the stable high symptomatology or improver classes. Age was not associated with trajectory class, but race and education were relevant. Whites were 56% more likely than African Americans to be in the stable high symptomatology class than in the stable low symptomatology class. Race, however, did not distinguish either the decliner or improver classes from the reference group. Increased education reduced the odds of being in the decliner, improver, or stable high symptomatology trajectory class. In summary, the only associations found between demographic characteristics and trajectory class of depression were that being male, and reporting more years of education increased the odds of being in the stable nondepressed class, while being White increased the odds of being in the stable high symptomatology class. None of the demographic characteristics distinguished among the improver, decliner, or stable high depressive symptomatology classes.
Health was predictive of trajectory class. The stable low depressive symptomatology class had significantly better self-rated health than any of the other classes. The odds of poorer self-rated health was about 50% greater in the improver class, double in the decliner class, and 2⅔ greater in the stable high symptomatology class. The functional status of the improver class did not differ significantly from that of the stable low depressive symptomatology class, but the functional status of the decliner and stable high symptomatology classes was 10% poorer. Poorer cognitive status uniquely increased the odds of being in the decliner class. Comparison of the improver, decliner, and stable high symptomatology classes indicated that the self-rated health of the improver class was significantly better than that of the stable high symptomatology, but there were no statistically significant differences across these three classes with respect to functional status or cognitive status.
Regarding social factors, stressful life events were strongly associated with trajectory class. Compared to the stable low symptomatology class, the odds of stressful life events were nearly double for the stable high symptomatology class, about 50% higher for the decliner class, and about 25% higher for the improving class. Larger network size was associated with increased odds for the improver class. Higher social interaction had conflicting effects, reducing the odds of both improving and declining. It did not distinguish the stable high symptomatology class from the stable low symptomatology class. Comparison across improver, decliner, and stable high depressive symptomatology classes indicated that while stressful life events were significantly higher in the improver than in the low depressive symptomatology class, they were nevertheless significantly lower in the improver class than in the stable high depressive symptomatology class. In addition, the improver class had a significantly larger network than the stable high depressive symptomatology class.
In summary, with the exception of age, all the variables examined distinguished the stable low depressive symptomatology class from one or more of the other three trajectory classes. Four variables consistently distinguished the stable low depressive symptomatology class from all of the other three classes: gender (a significantly higher proportion of men), increased education, better self-rated health, and fewer stressful life events. Of these variables, self-rated health and stressful life events also distinguished the stable high depression symptomatology class from the improver class, the latter being in a more preferred position than the former (better self-rated health, fewer stressful events). The stable high depressive symptomatology class was distinguished by race (greater odds of being white), but otherwise shared characteristics with at least one other trajectory class. This class was most likely to report poorer self-rated health, and had nearly twice the odds of experiencing stressful life events. The decliner class was distinguished by poorer cognitive status, but they also reported poorer self-rated health and functional status, and had an increased odds of stressful life events. The improver class reported a larger social network than any of the classes, and their self-rated health, and stressful life events were second only to that of the stable low depressive symptoms class.