What does all this mean for future prevalence estimates? Prevalence estimates of Alzheimer’s disease reported in future Facts and Figures reports will continue to report clinically diagnosed individuals in the dementia stage, currently estimated to be 6.2 million Americans, in addition to the best-available estimates of MCI due to Alzheimer’s disease. Facts and Figures will not include prevalence estimates for the preclinical Alzheimer’s disease stage until (1) there is conclusive 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 impacted. Ultimately, with these refinements, future prevalence estimates of Alzheimer’s disease will be higher, although estimates of Alzheimer’s dementia (the currently reported 6.2 million) will be lower. For example, in 2021, biomarker-based estimates of Alzheimer’s disease could result in up to 30% fewer individuals in the dementia stage of the disease, leaving roughly 4 million Americans age 65 and older with Alzheimer’s dementia, while also including an additional 50% of those over age 65 with MCI (those with MCI due to Alzheimer’s disease). A conservative estimate of the number of people with MCI due to Alzheimer’s disease is 5 million Americans. This would result in a very rough current estimate of 9 million Americans with Alzheimer’s disease. It should be noted that both symptom-based and biomarker-based prevalence estimates will rapidly increase in the future due to growth in the proportion of Americans age 65 and over, the population most at risk. When epidemiological studies include biomarker-based diagnoses and a conclusive connection is shown between biomarkers and the preclinical stage, an Alzheimer’s disease prevalence estimate ought to include individuals throughout the continuum of Alzheimer’s disease: those with biomarker-confirmed Alzheimer’s dementia, those with biomarker-confirmed MCI due to Alzheimer’s disease and those with preclinical Alzheimer’s disease — making the true prevalence of Alzheimer’s disease even higher. Prevalence 21 older patients for cognitive impairment are (1) the patient presents with no symptoms or complaints (68%) and (2) lack of time (58%). Prevalence of Subjective Cognitive Decline The experience of worsening or more frequent confusion or memory loss (often referred to as subjective cognitive decline) is one of the earliest warning signs of Alzheimer’s disease and may be a way to identify people who are at high risk of developing Alzheimer’s or other dementias as well as MCI.229-233 Subjective cognitive decline refers to self-perceived worsening of memory and other thinking abilities by an individual, separate from cognitive testing, clinical diagnosis or anyone else noticing. Not all those who experience subjective cognitive decline go on to develop MCI or dementia, but many do.234-236 One study showed those who over time consistently reported subjective cognitive decline that they found worrisome were at higher risk for developing Alzheimer’s dementia.237 The Behavioral Risk Factor Surveillance System survey, which includes questions on subjective cognitive decline, found that 11% of Americans age 45 and older reported subjective cognitive decline, but 54% of those who reported it had not consulted a health care professional.238 Individuals concerned about declines in memory and other cognitive abilities should consult a health care professional. Estimates of the Number of People with Alzheimer’s Dementia by State Table 4 lists the estimated number of people age 65 and older with Alzheimer’s dementia by state for 2020 (the most recent year for which these numbers are available), the projected number for 2025 and the projected percentage change in the number of people with Alzheimer’s between 2020 and 2025.A3,239 As shown in Figure 3, between 2020 and 2025 every state across the country is expected to experience an increase of at least 6.7% in the number of people with Alzheimer’s. These projected increases in the number of people with Alzheimer’s are based on projected increases in the population age 65 and older in these states. Because risk factors for dementia such as midlife obesity and diabetes can vary dramatically by region and state, the regional patterns of future burden may be different than reported here. Based on these projections, the West and Southeast are expected to experience the largest percentage increases in people with Alzheimer’s dementia between 2020 and 2025. These increases will have a marked impact on states’ health care systems, as well as the Medicaid program, which covers the costs of long-term care and support for many older residents with dementia, including more than a quarter of Medicare beneficiaries with Alzheimer’s or other dementias.240 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 these studies are in their infancies. 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.35 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.225-226 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.41-44 Furthermore, only about half of Medicare beneficiaries who have a diagnosis of Alzheimer’s or another dementia in their Medicare billing records report being told of the diagnosis.45-48,227 Because Alzheimer’s dementia is often underdiagnosed — and if it is diagnosed by a clinician, people are often unaware of their diagnosis — a large portion of Americans with Alzheimer’s may not know they have it. For more detailed information about detection of Alzheimer’s and other dementias in the primary care and clinical setting, see the Special Report from 2019 Alzheimer's Disease Facts and Figures. 228 Since 2011, the Medicare Annual Wellness Visit has included a required cognitive evaluation.