Hidden biology: Blood tests rewrite our understanding of Alzheimer's disease prevalence

Imagine walking through a busy town square in Norway. You see hundreds of older adults chatting, shopping, and navigating their day with ease. To the naked eye, they appear completely healthy. Yet, deep within the brain chemistry of nearly a quarter of the over-70s walking past you, a silent biological process has already begun. This hidden reality is what a new study sought to uncover using advanced blood analysis.

Historically, checking for the brain changes associated with Alzheimer's required expensive brain scans or invasive spinal taps. This limited our ability to see the big picture. In this study, researchers analysed plasma samples from 11,486 individuals. They measured a specific biomarker called p-tau217. If this protein is elevated in the blood, then it acts as a strong signal that plaques and tangles are accumulating in the brain.

New data on Alzheimer's disease prevalence

The study measured how common these biological changes really are. The results were stark. In participants aged 58 to 69, fewer than 8% showed signs of the disease. However, the numbers climb steeply with time. In those over 90 years of age, the estimated prevalence of these neuropathological changes hit 65.2%. It is a massive increase.

What makes this research particularly interesting is the distinction between biology and symptoms. If you have the biological markers but your memory is fine, you have 'preclinical' Alzheimer's. The data showed that among people over 70, roughly 10% fell into this silent category. Another 10.4% had early, mild symptoms, while 9.8% had developed dementia. Most surprisingly, in the group of people over 70 who were cognitively unimpaired—those with no memory complaints at all—23.5% still tested positive for the disease markers.

These measurements suggest that the biological footprint of the condition is much wider than the visible clinical cases. It implies that for many, the underlying mechanisms of the disease are active long before a doctor ever makes a diagnosis.