How common is Alzheimer’s? Blood-test study holds surprises

A recent study conducted in Norway, which combined clinical assessments with blood-based markers, indicates that almost 10% of individuals over the age of 70 are affected by Alzheimer’s disease dementia. This figure aligns with previous estimates for some white populations.

The research, published in *Nature*, also uncovered some unexpected variations, including a higher-than-anticipated prevalence of the disease in individuals over 85. According to Nicolas Villain, a neurologist at Sorbonne University in Paris, who was not involved in the study, this is a significant contribution from a well-designed Norwegian study. The findings suggest that blood-based tools can refine epidemiological estimates of neurodegenerative diseases.

While blood tests are now available for Alzheimer's, Jason Karlawish, a geriatrician and co-director of the Penn Memory Center in Philadelphia, Pennsylvania, cautions that the optimal use of these tests remains a subject of debate. Karlawish, who also did not participate in the study, suggests that while blood-based markers can be valuable for physicians treating dementia patients and for research purposes, they are not yet suitable for widespread use as general health screening tools. He warns that the misuse of such tests could potentially cause harm.

To evaluate the prevalence of Alzheimer's disease (AD), a team of international researchers analyzed data from the Trøndelag Health (HUNT) study. This ongoing prospective health research initiative, which began in 1984, has gathered health information and biological samples from 250,000 Norwegians.

The researchers examined blood samples from 11,486 participants aged 58 and older, focusing on levels of phosphorylated tau protein (pTau217). This specific blood marker serves as an indicator of amyloid plaque buildup in the brain, a characteristic feature of AD. Cognitive testing was performed on HUNT study participants over 70, enabling researchers to correlate pTau217 levels with the presence of dementia.

The study revealed that approximately 10% of participants over 70 exhibited both dementia and AD pathology, indicated by cognitive impairment and elevated pTau217 levels. Another 10% showed mild cognitive impairment along with high pTau217 levels, while an additional 10% had high pTau217 levels but no apparent cognitive impairment, a condition the authors termed preclinical AD.

Although these results generally matched expectations, some unexpected findings emerged. Among individuals aged 85–89, around 25% had dementia and AD pathology, which is higher than previous estimates of approximately 7% for men and 13% for women in this age group in Western Europe. Conversely, the incidence of preclinical AD in younger individuals (aged 70–74) was 8%, lower than a previous estimate of about 22%.

Anders Gustavsson, an advisor to the health economics consultancy Quantify Research in Stockholm and a member of the team that produced the earlier estimates, welcomes the new data. He stated that he is not surprised by the differences in the numbers obtained in this study.

Study co-author Anita Lenora Sunde, a physician and dementia researcher at Stavanger University Hospital in Norway, suggests that selection bias may account for these discrepancies. Prior estimates involved recruiting participants for brain scans, and individuals with dementia may have been less willing or able to participate. Nicolas Villain adds that the high pTau217 threshold used in the latest study to define preclinical AD may exclude individuals with intermediate protein levels and emerging pathology. Lowering this threshold, he notes, would likely increase the estimated prevalence.

Furthermore, the study indicated that individuals with lower levels of education tend to have higher pTau217 levels.