longitudinal studies when determining prevalence rather than one study.A3,228 Second, as measurements of the brain changes of Alzheimer’s disease are more widely used, we will be able to estimate how many people have Alzheimer’s disease (not just dementia due to Alzheimer’s disease), which includes people in the earliest detectable stages of Alzheimer’s who have the brain changes of Alzheimer’s but not the overt symptoms of Alzheimer’s that interfere with their ability to carry out everyday activities. For decades it has been recognized that all individuals with dementia pass through a precursor stage frequently referred to as mild cognitive impairment (MCI). More recently, with the advent of biomarkers that detect abnormal levels of the betaamyloid and tau proteins that characterize Alzheimer’s, it is now possible to determine which individuals diagnosed with MCI have MCI due to Alzheimer’s disease. As biomarker-based diagnoses become more common, individuals with MCI due to Alzheimer’s disease will be included in prevalence estimates of the number of Americans with Alzheimer’s disease. As reported in this section, the limited data available to date show that about half of individuals age 65 and older with MCI — roughly 5 million Americans — have MCI due to Alzheimer’s disease. Because MCI develops years before dementia and potentially affects individuals younger than 65, there are likely far more than 5 million Americans — of any age — with MCI due to Alzheimer’s disease. Last, the National Institute on Aging – Alzheimer’s Association (NIA-AA) Framework232 hypothesizes that there is an incipient and silent stage of Alzheimer’s disease before the emergence of cognitive symptoms of either MCI or dementia. While this is still the subject of additional research to better understand these early brain changes, estimates have begun to be made regarding the prevalence of “preclinical” Alzheimer’s disease in the population.233 More research is needed to validate preclinical Alzheimer's and determine how to measure it with biomarkers that conclusively represent Alzheimer’s disease, as opposed to other dementia-causing diseases. What does all this mean for future prevalence estimates? Future Facts and Figures reports will continue to include the estimated prevalence of individuals in the dementia stage, defined according to clinical symptoms only, currently estimated at 6.5 million Americans, in addition to the best available estimated prevalence of MCI due to Alzheimer’s disease. Facts and Figures will not include prevalence estimates of the preclinical Alzheimer’s disease stage until (1) there is convincing evidence of a connection between biomarkers in this silent stage and the development of MCI due to Alzheimer’s disease and (2) prevalence studies have then attempted to calculate the number of individuals in this stage. In addition, as the evidence and epidemiological data warrant, future reports may also include estimates of the prevalence of dementia from all causes. Prevalence 21 The estimated number of people age 65 and older with Alzheimer’s dementia comes from an updated study using the latest data from the 2022 projections from the U.S. Census Bureau and the Chicago Health and Aging Project (CHAP), a population-based study of chronic health conditions of older people.224 National estimates of the prevalence of all dementias are not available from CHAP, but they are available from other population-based studies including the Aging, Demographics, and Memory Study (ADAMS), a nationally representative sample of older adults.A3,226-227 Based on estimates from ADAMS, 11% of people age 65 and older in the United States have dementia.228 Mild Cognitive Impairment due to Alzheimer’s Disease The number and proportion of older adults who have MCI due to Alzheimer’s disease is currently difficult to estimate because it requires population-based prevalence measures of MCI and Alzheimer’s biomarkers, and this line of research is in its infancy. Furthermore, there is variation across studies in both the threshold of cognitive impairment required for an MCI diagnosis and the level of biomarker burden that defines the presence of Alzheimer’s disease. However, we can roughly estimate this prevalence indirectly using multiple data sources. A systematic review of more than 30 studies of MCI reported that 16.6% of people age 65 and older had MCI.48 Meanwhile, studies assessing biomarkers for Alzheimer’s disease with PET scans have reported that about half of people with MCI have Alzheimer’s-related brain changes.234-235 Therefore, roughly 8% of people age 65 and older — or approximately 5 million older Americans — may have MCI due to Alzheimer’s disease. This prevalence estimate needs to be confirmed with population-based studies involving biomarkers. Underdiagnosis of Alzheimer’s and Other Dementias in the Primary Care Setting Prevalence studies such as CHAP and ADAMS are designed so that everyone in the study undergoes evaluation for dementia. But outside of research settings, a substantial portion of those who would meet the diagnostic criteria for Alzheimer’s and other dementias are not diagnosed with dementia by a physician.236-241 Furthermore, only about half of Medicare beneficiaries who have a diagnosis of Alzheimer’s or another dementia in their