Alzheimer’s disease (AD) may be more common than previously thought in some demographic groups, according to the first population-based study to use a blood test to gauge AD-related neuropathological changes (ADNCs).

The study found a higher prevalence of AD dementia in older individuals and a lower prevalence of preclinical AD in younger age groups, compared with previous studies with smaller, less representative cohorts, investigators said.

Notably, more than 1 in 10 adults older than 70 met the criteria for monoclonal antibody therapy that could potentially slow cognitive decline. 

A “major advance” of this study is the use of blood-based biomarkers as a way to generate more accurate, population-level estimates of AD pathology, Nicholas Ashton, PhD, senior director of the Banner Fluid Biomarker Program, Banner Sun Health Research Institute in Sun City, Arizona, told Medscape Medical News.

“It's really kind of taking a step away from the clinic and saying we can use these blood tests to better inform us about population-level prevalence of Alzheimer’s disease pathology across the board,” Ashton explained. 

The study was published online on December 17 in Nature.

More Accurate Assessment?

The researchers measured pTau217, a highly accurate blood biomarker for early AD pathology, in blood samples from 11,486 adults aged 58 or older enrolled in the HUNT study in Norway.

Participants were classified as having AD pathology, no AD pathology, or an intermediate result using validated cutoffs. All participants aged 70 or older underwent cognitive assessments to link blood biomarker findings to clinical diagnoses such as mild cognitive impairment (MCI) and dementia.

There was a stepwise increase in the prevalence of ADNCs with age — from less than 8% in adults aged 58-69, to 18% in those aged 70-74, 58% in those aged 85-89, and 65% in those aged 90 or older.

Among adults aged 70 or older who underwent cognitive assessment, 10% had preclinical AD (brain pathology without cognitive symptoms), 10.4% had prodromal disease (pathology plus MCI), and 9.8% had AD dementia.

In this 70+ age group, ADNCs were found in 60% of individuals with dementia, in 33% of those with MCI, and in 23.5% of those with no cognitive impairment.

“Our study used a simple blood test to establish changes that contribute to cognitive impairment in those with dementia. In doing so, we found that around 11% of participants over the age of 70 meet the eligibility criteria for monoclonal antibody treatments that can potentially slow the impact of cognitive decline in these individuals,” co-lead author Dag Aarsland, MD, PhD, professor of old-age psychiatry at King’s College London in London, England, said in a statement.

“If we are to meet this global challenge, it is vital that we are able to detect signs of Alzheimer’s disease at the earliest possible stages. This blood test looks to be an effective means of providing that clarity at scale,” Aarsland added.

Notably, the researchers said that based on plasma pTau217 concentrations below the lower cutoff, ADNCs were ruled out in 41% of the MCI group and 19.4% of the dementia group. Thus, in these groups, cognitive impairment is probably due to causes other than AD.

ADNCs were more common in people with lower educational attainment, especially in the older age groups, and among carriers of the APOE epsilon 4 gene variant, a known genetic risk factor. Men and women had similar rates of ADNCs across age groups.

The researchers cautioned that these findings are relevant to a mainly white Norwegian population. Because dementia varies across different ethnic groups, the prevalence of ADNCs may also differ in other populations, they noted.

Study Raises ‘Important’ Questions

Two outside experts shared their perspectives on this analysis with the UK nonprofit Science Media Center.

Louise Serpell, DPhil, emerita professor of biochemistry at the University of Sussex in Brighton and Hove, England, said it provides “some interesting and some surprising results.”

The fact that a number of individuals diagnosed with “dementia” were negative for pTau217 was “notable,” raising “important questions about the specific diagnosis and the reach of the marker,” Serpell commented.

Another notable finding was the lack of any significant difference in prevalence of ADNCs between sexes, “which raises questions about the popular belief that women are at a higher risk of Alzheimer’s disease,” Serpell said. 

Also weighing in, Tara Spires-Jones, PhD, director of the Centre for Discovery Brain Sciences at The University of Edinburgh, Edinburgh, Scotland, said the study adds to other data suggesting that AD prevalence is rising as the population ages.

She noted, however, that the prevalence figures are not that different from two other recent reports.

The observation in this study from Norway that around 60% of people over age 85 had ADNCs is “similar to a large review of data from around the world published in 2023, which found that 55%-59% of people over age 85 exhibited these changes,” Spires-Jones noted.

In addition, the finding in the Norway study that about 25% of people aged 85-89 had dementia is in line with data from the Alzheimer’s Association in 2025, which found that over 30% of people over age 85 have AD.

For these reasons, these new data are “not a surprise,” Spires-Jones noted.

She agreed with the authors that a key limitation was the relatively homogenous population in Norway with few people from diverse genetic backgrounds — an “important” caveat given that AD prevalence is known to be different in people from different ethnicities.

The study had no commercial funding. Ashton reported having given lectures, produced educational materials, and participated in educational programs for Eli-Lilly, BioArctic, and Quanterix, which markets a pTau217 test. A complete list of author disclosures is available with the original article. Serpell is the founder of Serpell-Science, a consultancy that advises on communication and research strategy for neurodegenerative diseases. Spires-Jones reported no relevant disclosures.