Blood Test Links Alzheimer's Proteins to Cognition Decades Before Old Age, Lancet Study Finds

A community based study of 1,350 adults in their 50s and 60s found that plasma markers of Alzheimer's disease neuropathology tracked with weaker cognitive performance and faster decline in midlife, long before the age when the disease is usually diagnosed.

People in their 50s and 60s who carried elevated blood markers of Alzheimer's disease neuropathology, none of them diagnosed with dementia, scored worse on tests of processing speed and executive function and were three to four times more likely to show accelerated cognitive decline over five years, according to a cohort study published in The Lancet on May 30, 2026. The finding, reported by Xiaqing Jiang and colleagues, is among the first to link plasma Alzheimer's biomarkers to cognition at midlife in a diverse, community-based population rather than an older clinical sample.

The researchers drew on the Coronary Artery Risk Development in Young Adults (CARDIA) study, a US cohort recruited in the mid-1980s and followed for decades. Blood drawn at CARDIA's year 35 exam, in 2020 to 2022, was tested for amyloid beta and phosphorylated tau, the two proteins that build up in the brains of people with Alzheimer's disease. After excluding participants with poor sample quality, missing cognitive data, or probable dementia, 1,350 people remained, with a mean age of 61 (range 53 to 69), 58 percent women and 45 percent Black. Using cutpoints validated against amyloid PET scans, the researchers classified 6 percent of the group, 86 people, as biomarker positive.

The study is observational: it shows an association between blood biomarker levels and cognitive test scores and change over time, not that the biomarkers caused the decline or that any participant will go on to develop Alzheimer's disease. Alzheimer's disease neuropathology was described by the authors as relatively uncommon in midlife overall, and an accompanying Lancet commentary by Anna Rosenberg and Tiia Ngandu, published alongside the paper, situates the results as evidence for how early such biology can be detected rather than as a diagnostic tool ready for clinical use.

The National Institutes of Health, which helped fund the CARDIA study through the National Heart, Lung, and Blood Institute and the National Institute on Aging, featured the finding in a September 2026 issue of its consumer publication News in Health, describing it as evidence that a blood test might one day flag early brain changes tied to Alzheimer's disease. The write-up, like the paper itself, stopped short of recommending testing or treatment for people without a diagnosis.

The authors say the CARDIA cohort will continue to be followed, which should show whether the biomarker differences seen at midlife predict who develops dementia decades later.