In this study, we report prevalence estimates of CIDI SF-diagnosed major depression in the U.S.A. of 11.19% among individuals aged over 70. We found rates of depression based on sex, age, and ethnicity that merit further discussion. While women had a higher prevalence of CIDI-depression, the female-to-male ratio was 11.44%:10.81%, or approximately 1.06. Older men with dementia had depression by NPI report with nearly three times the frequency than older women with dementia. In terms of age, prevalence of depression was over 12% among those aged 80 and older, while septagenerians had an overall prevalence of 10.38%. For the three ethnic groups examined, depression rates were about three times higher among non-Hispanic whites and Hispanics compared with non-Hispanic blacks, the latter having a prevalence rate of about 4%. Finally, depression prevalence progressively increased with worsening cognitive impairment (from those with no cognitive impairment to cognitive impairment without dementia to dementia).
The depression prevalence rates across demographic variables (sex, age, and ethnicity) are the most striking and perhaps most surprising results of the study. Our finding of similar depression prevalence for men and women is at variance with prior studies that found higher rates of depression among women than men (Regier et al., 1988
; Steffens et al., 2000
). The difference may be explained in part by the populations sampled in our study versus other studies. For example, the Cache County study included a homogeneous population of elders in northern Utah (Steffens et al., 2000
). ADAMS is a more population-representative sample, so factors related to diversity of ethnicity and of socio-economic status (Murrell et al., 1983
; Black et al., 1998
) may equalize prevalence between the sexes. Prior studies found a lower prevalence of depression among the elderly compared with other age groups; our inclusion of cognitively impaired individuals may in part explain a prevalence rate that is higher compared with community-dwelling elders, as well as our finding of higher prevalence for individuals aged 80 and older compared with the rate in individuals in their 70s. Finally, the null sex effect may be due to sample size as evidenced by the rather broad confidence intervals for the sex odds ratio.
Previous studies have reported lower rates of depression among non-Hispanic blacks generally (Williams et al., 2007
) and in older adult populations in particular (Grunebaum et al., 2008
). There have been variable reports for prevalence of depression among Hispanics with some studies reporting rates comparable to non-Hispanic whites (Williams et al., 2007
), and others showing lower rates (Riolo et al., 2005
). Interestingly, one study suggested an age effect such that among those in the older cohort, there were no significant differences in risk for mood disorders between Hispanics and non-Hispanic whites, but in the younger cohort, Hispanics had significantly lower risk than non-Hispanic whites for mood disorder (Breslau et al., 2006
Our finding of greater prevalence of NPI-diagnosed depression among cognitively impaired individuals and those with dementia compared with those with no cognitive impairment is also consistent with previous research. For example, one study reported a high prevalence of mood symptoms among patients with “cognitive impairment, no dementia” (Peters et al., 2008
). In a Brazilian cohort, 16% of those with cognitive impairment, no dementia had NPI-rated depression (Tatsch et al., 2006
). Among populations with dementia, prevalence of depression is high. One study of four Alzheimer’s Disease Research Center populations found a prevalence of major depression of 22.5% to 54.4% across recruitment sites (Zubenko et al., 2003
). Major depression is common among patients with mild (11.5%) and moderate (10%) AD, but occurs at a lower rate in severe AD (4.5%) (Lopez et al., 2003
). Depression symptoms are also common in patients with other forms of dementia, including vascular dementia (Sultzer et al., 1993
; Park et al., 2007
), dementia with Lewy bodies (Klatka et al., 1996
; Papka et al., 1998
; Ballard et al., 1999
), Parkinson’s disease (Tandberg et al., 1996
; Weintraub and Stern, 2005
), and Huntington’s disease (Folstein et al., 1983
). Finally, we found that depression prevalence increased with age when NPI depression data were used.
Major strengths of this study include a population-representative sample that allowed estimation of depression prevalence in the U.S. population by sex and across age and ethnic/racial groups. Another important feature is that the majority of the sample is community-based, with 718 of 856 assessments having been completed in the respondent’s home. In addition, the study included both normal and cognitively impaired individuals in the same population representative sample, which facilitates comparison of depression prevalence in the presence and absence of clinically diagnosed cognitive impairment. Another strength is the connection of this study to the larger Health and Retirement Study, an established cohort of individuals with well-documented longitudinal demographic social, economic and health data. By using the wealth of data available from the HRS, we are able to adjust for non-response in the sample. In fact, this study extends previous work that utilized the HRS data to examine depression prevalence among middle-aged and older adults, citing a 12-month prevalence of major depression of 6.6% (Mojtabai and Olfson, 2004
). Unlike the present study, it does not appear that this previous HRS paper included individuals in the depressed group if they indicated that they were not depressed because of antidepressant use. The ability to combine the HRS data with depression and cognitive data in ADAMS provides a unique perspective on depression across a variety of socio-demographic strata and across a spectrum of cognitive function.
This study also has potential limitations worth noting. For example, despite the attempt to ensure a population-representative sample, the size of certain subgroups within the population was too small to estimate prevalence reliably. These included black and Hispanic males for the population overall. In addition, the small number (N = 76) of individuals from whom we had NPI informant reports for depression precluded estimates of depression among blacks and Hispanics.
Methodology related to our depression assessment may also represent a limitation for the study. We relied primarily on self-report structured interviews using the CIDI-SF, an instrument that has been shown to be valid in epidemiological studies, but may nonetheless fall short of diagnoses achieved through clinical interview. One advantage of our method is that we were able to capture those individuals with depressive disorder
with low report of depressive symptoms
due to current depression treatment. It is likely that combining individuals with depressive symptoms and those who report treatment is a strategy that provides a more accurate measure of depressive disorder (Norton et al., 2006
). However, our inability to determine whether reported depressive treatment was for major or minor depression forced us to collapse categories of major and minor depression into a single category of depression. It is also possible that the estimates of the frequency of depression treatment in the present study may be a slight underestimate of treatment given the algorithm in which participants reported treatment only if they indicated that they had no depressive symptoms due to receiving antidepressant treatment, and informants were only asked about treatment if they endorsed depressive symptoms on the NPI.
While inclusion of cognitively impaired individuals provides some advantages in terms of estimation of depression prevalence among all older adults, the methodological approach presents limitations as well. Our reliance on the NPI, an instrument whose performance characteristics in diagnosing syndromal depression is understudied, may provide a faulty prevalence estimate of major or minor depression in this population. Further, our method of combining CIDI-SF and NPI data to estimate overall depression prevalence must be mentioned among study limitations. These two instruments differ in several respects: subject-based versus informant-based, wording, and time frame of depressive symptoms. Combining the two scales thus may limit our ability to provide reliable diagnostic estimates. While lack of diagnostic clarity is a clear limitation, we believe that our strategy does provide a good estimate of clinically significant depression among a nationally representative sample of older Americans. However, the inclusion of depression based on antidepressant treatment data in our classification scheme may explain why our one-month depression prevalence is substantially higher than the 6.6% one-year prevalence previously reported in the HRS sample (Mojtabai and Olfson, 2004
A final potential concern is our opportunistic approach of estimating depression prevalence within the context of a study (ADAMS) whose primary aim is to characterize cognition and cognitive decline in an older population. Depression instruments in ADAMS were designed mainly to facilitate cognitive assessment. If depression were the major aim of the study, we would likely have chosen measures with greater diagnostic accuracy such as the Structured Clinical Interview for DSM-IIIR (SCID; Williams et al., 1992
). We discuss limitations of the instruments above. Another potential concern is that our prevalence estimates of depression may have been influenced by the oversampling in ADAMS of individuals with cognitive impairment, a group with increased risk for depression (Steffens and Potter, 2008
). Such bias is unlikely because the complex sample weights used to estimate the depression prevalence were constructed to account for this issue.
In sum, the ADAMS cohort provides a population-based estimate of depression among older Americans. We found surprisingly similar depression prevalence between men and women. Non-Hispanic whites and Hispanics had much higher depression prevalence than blacks. Individuals with dementia had among the highest prevalence of current depression, which highlights the need for additional study on the social impact of this comorbidity. Future studies may also need to oversample among certain ethnic groups, e.g. black and Hispanic men, in order to obtain reliable depression estimates for these populations.