New Blood Test May Detect Alzheimer's 25 Years Before Symptoms Appear

What if it were possible to predict the risk of dementia decades before the first signs appear? A simple blood test may be opening that door. Scientists have discovered that elevated levels of a specific protein in the body are linked to up to three times the risk of developing dementia, and most impressively: this can be detected up to 25 years before symptoms appear. The discovery promises to transform how we understand, prevent, and even treat neurodegenerative diseases, taking into account factors such as age, genetics, and lifestyle.

Dementia, including Alzheimer's disease, is one of the conditions that most concern the aging population. One of the biggest challenges has always been identifying who is at risk before symptoms appear. Now, a new study suggests that this may be possible with a simple blood test, capable of indicating the risk of cognitive problems up to 25 years before they arise.

This test measures a substance present in the blood called p-tau217, a protein linked to brain changes associated with Alzheimer's disease. When this protein appears at higher levels, it may indicate that changes in the brain are already beginning, even if the person does not yet have any signs of memory loss or difficulty thinking.

To investigate this relationship, researchers followed approximately 2,766 elderly women in the United States for up to 25 years. At the beginning of the study, all were cognitively healthy, meaning they showed no signs of dementia or mild memory impairment.

The tests were performed using blood samples collected from elderly women back in the 1990s, when all showed no signs of memory problems. This blood was stored for many years and, decades later, analyzed using modern techniques capable of detecting very small amounts of a protein called p-tau217, associated with Alzheimer's disease.

Next, the researchers compared the levels of this protein with what happened to the participants over up to 25 years, observing who developed cognitive impairment or dementia. This allowed them to verify that higher levels of this substance in the blood were linked to a greater future risk, showing that changes related to the disease can be identified long before the onset of symptoms.

The results showed that higher levels of this protein were strongly associated with the future risk of developing cognitive problems. In total, about 47% of the participants (1,311 out of 2,766) developed some degree of cognitive impairment or dementia during the follow-up. This allowed the researchers to identify clear patterns between the blood test and the risk over time.

When analyzed in detail, the increase in this protein in the blood was linked to a significantly higher risk:

- For any cognitive problem (mild or dementia): approximately 2.4 times higher risk.

- For mild memory impairment: approximately 1.9 times higher risk.

- For dementia: the risk is approximately 3.2 times higher.

In simple terms, this means that the higher the level of this protein, the greater the likelihood of a person developing cognitive problems in the future, especially dementia. The study also showed that this risk is not the same for everyone. It was higher in some specific groups:

- Women over 70 years old.

- People with a specific genetic predisposition (known to increase the risk of Alzheimer's).

- White women, compared to Black women.

- Women who underwent a specific type of hormone therapy (a combination of estrogen and progestin).

On the other hand, in Black women, the protein did not show a clear relationship with mild memory impairment, indicating that other factors may influence the risk in this population.

Despite the promising results, the researchers emphasize that this test is not yet a definitive diagnosis. It serves as a risk indicator, helping to identify people who may need closer monitoring in the future. Further studies are still needed to understand how to use this information in clinical practice.

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Plasma Phosphorylated Tau 217 and Incident Mild Cognitive Impairment and Dementia in Older Women

Aladdin H. Shadyab, Bowei Zhang, Andrea Z. LaCroix, Michelle M. Mielke, Susan M. Resnick, Steve Nguyen, Luigi Ferrucci, Towia A. Libermann, Long Ngo, Ramon Casanova, Alexander P. Reiner, Danni Li, Caroline M. Nievergelt, Adam X. Maihofer, JoAnn E. Manson, and Linda K. McEvoy

JAMA Network Open. 10 March 2026;9;(3):e261295.DOI: 10.1001/jamanetworkopen.2026.1295

Abstract:

There is limited research on the long-term associations of plasma phosphorylated tau 217 (p-tau217) with mild cognitive impairment (MCI) and dementia. No study has evaluated whether such associations vary by race or hormone therapy (HT) use. To examine associations of baseline plasma p-tau217 with incident MCI and dementia and determine whether associations vary by age, race, APOE ε4 carrier status, or HT use. This cohort study examined women recruited from 39 US clinical sites between 1996 and 1999 into the Women’s Health Initiative Memory Study who were randomized to either estrogen alone vs placebo or estrogen plus progestin vs placebo. Women were assessed for up to 25 years through 2021. Baseline plasma p-tau217 was measured in 2024 and analyzed between February and August 2025. Women aged 65 to 79 years who were cognitively unimpaired at baseline were included for this analysis. Plasma p-tau217, quantified using the ALZpath Simoa assay. The primary outcome was the combined end point of incident MCI or probable dementia. Secondary outcomes included MCI and dementia examined separately. Cause-specific hazard ratios (HRs) and 95% CIs for the association of p-tau217 with MCI or dementia were estimated using Cox proportional hazards regression models. Among 2766 participants (mean [SD] age, 69.9 [3.8] years; 486 [17.9%] Black, 196 [7.1%] Hispanic, and 2007 [73.9%] White), 1311 developed the combined end point of MCI or dementia (849 participants with MCI and 752 participants with dementia). Every 1-SD increase in log2-transformed p-tau217 was associated with incident MCI or dementia (HR, 2.43; 95% CI, 2.18-2.71) and each individual outcome (MCI: HR, 1.94; 95% CI, 1.72-2.20; dementia: HR, 3.17; 95% CI, 2.79-3.61). Associations of p-tau217 with dementia were larger in magnitude for women randomized to estrogen plus progestin (HR, 4.18; 95% CI, 3.41-5.13) vs placebo (HR, 3.07; 95% CI, 2.41-3.91) (P for interaction = .04) but did not significantly vary by estrogen alone vs placebo. P-tau217 associations with MCI or dementia were larger in magnitude for women older than 70 years (P for interaction = .04), APOE ε4 carriers (P for interaction = .02), and White women compared with Black women (P for interaction < .001). However, the combination of p-tau217 and age performed similarly in White women (area under the curve = 72.0%; 95% CI, 70.3%-73.6%) and Black women (area under the curve = 70.4%; 95% CI, 64.0%-78.0%). P-tau217 was not associated with incident MCI in Black women.  In this cohort study of cognitively unimpaired older women, p-tau217 was associated with incident MCI or dementia up to 25 years later. These findings suggest that age, race, APOE ε4, and HT use should be considered when examining associations of p-tau217 with cognitive outcomes.