The ADAMS has produced the first prevalence estimates of dementia and AD in a nationally representative sample in the USA to include individuals from all regions of the country. To allow comparison with findings from previous studies using a lower minimum age (i.e. either age 60+ or 65+), we combined the estimates from ADAMS for ages 71 and greater with those from other studies for ages 60–70 [3
]. This resulted in an estimated total of 3.8 million individuals with dementia and just over 2.5 million with AD in the USA. The sole previous national estimate of dementia prevalence was 2.9 million, based on a Delphi consensus review of previously published studies in the USA [3
]. The four previous national estimates of AD prevalence differed by greater than twofold and ranged from 2.1 million [6
] to 4.5 million [5
]. The lowest estimate came from a meta-analysis of 18 US and European studies, the highest from the East Boston and Chicago community studies [5
]. Variability in prevalence estimates of AD due to geographic factors has been discussed. In addition to the issue of extrapolation from regional samples, one likely source for variation among AD prevalence estimates is the use of different criteria for dementia. Some studies used criteria that do not require evidence of impaired functional performance [27
], while most use criteria requiring significant impairment in social or occupational functioning [17
]. Another likely source of study variation is the use of different methods to identify the ‘border’ between cognitive impairment that is not severe enough to meet criteria for dementia. This intermediate state between normal cognitive function and dementia is often referred to as CIND [28
] or mild cognitive impairment [29
]. Future analyses of ADAMS data, including analyses of longitudinal follow-up assessments of those diagnosed with CIND, will be important to help clarify the border between CIND and dementia in population-based settings.
Comparisons of prevalence estimates across studies are also difficult due to differences in the age brackets reported. However, a general comparison of age-specific prevalence rates from the ADAMS with those from local and regional samples in the USA [30
] suggests that, as a group, findings from the other studies span the estimates produced in the ADAMS, possibly reflecting its more complete representation of the US population. The completion of the ADAMS will also facilitate international comparisons of dementia prevalence between other countries and the USA. A comparison with the Canadian population [34
] shows that the two countries have similar rates of dementia.
Few predictors of AD and other dementias have been consistently identified across studies. One explanation often cited for these incongruent results is the lack of a sufficient sample size spanning the variable range. On this point, the ADAMS representative sample likely has advantages. Not surprisingly, age was the strongest predictor of both AD and other dementias in the ADAMS. Consistent with several [for a review, see [35
]], but not all [37
] other studies, more years of education were associated with a lower risk of dementia. Several studies have reported that females are at greater risk of AD than males [34
]; however, others have reported no such difference [40
]. In the ADAMS, women were not at higher risk for AD and other dementias. Results have been discrepant from the few regional studies that have examined race as a predictor for dementia. Some studies reported a higher frequency of AD or dementia among African Americans compared to Caucasians [42
], while another reported no such difference [44
]. In the ADAMS, African Americans had a higher frequency of dementia and AD, but once education, gender and APOE genotype were controlled, the OR was still elevated, but no longer statistically significant. These findings were similar to those from two other studies [45
]. Consistent with many other studies [47
], we found that the APOE
4 allele was associated with an increased risk of AD and dementia in general.
This study has several strengths: a representative, directly assessed sample of the US population aged 71 and older; the inclusion of large numbers of individuals with few years of education; a sizeable sample over the age of 90, and the inclusion of long-term care residents. All of these groups have a high prevalence of dementia. In addition, employing a single, experienced assessment team, successfully used in other population studies, and one common expert case review panel likely minimized diagnostic variability.
Some limitations also exist. The ADAMS participation rate was lower than hoped for but comparable to other population studies of this age group, such as the Cardiovascular Health Study (participation rate of 57.3% [48
]) and the Canadian Study of Health and Aging (68.5% [34
]). Both studies have made major scientific contributions to our understanding of health and memory in late life. Nonparticipation in all such studies could result in selection bias. The ADAMS has addressed potential nonresponse bias using detailed archived information from prior interviews, although models based on measures collected 6–18 months prior to the ADAMS assessment may not fully capture selection bias. However, given the range of available measures, it is likely that the response propensity models and the associated weighting adjustments do capture the major factors that could contribute to any significant selection bias in population estimates based on the ADAMS data. We also note that the lack of neuroimaging and other medical tests for all participants may have influenced the accuracy with which non-AD dementias were identified. However, for the 39% of individuals with dementia for whom neuroimaging results were available, in no instance was a solely non-VaD diagnosis that was assigned prior to review of neuroimaging subsequently changed to a solely VaD diagnosis after review of these records. Finally, our supplemental analyses grouping those with ‘dementia, undetermined etiology’ with the AD group may somewhat overestimate the prevalence of AD. Our previous research [22
] justifying this analysis included only Caucasian subjects. Although 16 of the 23 individuals with ‘dementia, undetermined etiology’ in the ADAMS were Caucasian, it is not clear whether these findings would generalize to the minority of African Americans with this diagnosis in the ADAMS.
As the elderly US population grows, the number of individuals with dementia will also increase, making planning for the long-term care needs of these individuals increasingly important. The value of the ADAMS, the first study of dementia in a nationally representative sample in the USA, extends beyond just estimating the prevalence of dementia to being able to address many of the key questions in preparing for the care of the demented and their families. These prevalence estimates provide the framework necessary to assess the impact of treatment advances as they become available [49
]. In the years to come, the ADAMS methodology can provide a marker of how well the country is doing with respect to the control and treatment of AD and other dementias. Regional studies in the USA will now have a national estimate with which to compare when exploring regional differences in disease patterns. The ADAMS data can also be enriched with other data collected from the ongoing HRS [14
] and as part of the linkage of HRS to Medicare records allowing researchers to explore questions that might increase our understanding of, and ability to successfully address, the needs of an aging US population.