A blood test could detect Alzheimer’s disease decades before symptoms appear, according to two new studies that mark a significant step towards earlier diagnosis and potential treatment. The research, published in The Lancet, suggests that measuring specific proteins in the blood can identify the biological hallmarks of Alzheimer’s in midlife, when cognitive decline may still be subtle and reversible.
The core of the breakthrough lies in the ability to detect so-called biomarkers — proteins that accumulate abnormally in the brain long before memory loss or confusion become apparent. Alzheimer’s disease is characterised by the build-up of amyloid-beta peptides, which form sticky plaques, and tau proteins, which twist into tangles inside neurons. Until recently, these could only be measured reliably through expensive and invasive procedures such as lumbar punctures to sample cerebrospinal fluid, or by PET scans that use radioactive tracers.
Now, ultrasensitive blood tests — using technology such as single molecule array (Simoa) assays — can pick up vanishingly small quantities of these proteins in the bloodstream. Particular attention is being paid to a form of tau called p-tau217, which has been shown to correlate closely with amyloid plaque burden in the brain as measured by PET scans. Researchers believe that p-tau217 may be more specific to Alzheimer’s than other biomarkers, and that its levels change early in the disease process. The ratio of p-tau217 to another amyloid marker, Aβ42, is also emerging as a powerful diagnostic signal.
What the blood test study found
In the first study, scientists analysed blood samples from 1,350 people in the United States who did not have dementia at the outset. Their average age was 61. Among them, 86 individuals were found to have high levels of two amyloid biomarkers and p-tau217. Over a five-year follow-up period, those with elevated biomarkers performed worse on cognitive tests — they showed poorer verbal memory, slower processing speeds, and a faster rate of decline compared with their peers.
The researchers noted that these findings build on earlier work in older populations by “showing that evidence of Alzheimer’s disease neuropathology is present in midlife, although infrequent, and is already linked to measurable cognitive differences”. They concluded that “these findings support the concept that Alzheimer’s disease begins decades before clinical symptoms emerge and highlight the potential value of plasma biomarkers for early detection in the general population”.
Importantly, the study also underlines midlife as a critical window for intervention. Subtle cognitive changes — particularly in processing speed and executive function — may be the earliest signs of the disease, emerging before more obvious memory problems.
A more sensitive brain scan
A separate study in the same journal examined a different approach: improving PET scans to detect tau tangles before symptoms appear. The researchers compared two radioactive tracers — Flortaucipir (marketed as Tauvid and licensed in the UK, though not routinely available on the NHS) and an investigational agent known as MK6240. In a trial involving 682 patients across the United States and Canada, MK6240 identified more than twice as many tau-positive cases in early tau regions compared with Flortaucipir. This suggests that newer tracers could pick up Alzheimer’s-related changes at an even earlier stage, potentially enabling more precise selection of participants for clinical trials and earlier diagnosis.
What this means for the UK
Dementia currently affects an estimated one million people in the United Kingdom, and that number is projected to rise to 1.4 million by 2040. Alzheimer’s disease accounts for between 60% and 80% of all cases and is the country’s biggest killer. Yet despite its prevalence, many people wait far too long for a diagnosis. At present, only about 2% of those diagnosed with Alzheimer’s have access to gold-standard diagnostic tests such as PET scans or lumbar punctures.
Dr Jacqui Hanley, head of research funding at Alzheimer’s Research UK, said the two studies “add to a growing body of evidence showing progress in detecting the biological changes linked to Alzheimer’s disease much earlier in life, using a range of biomarkers from blood tests through to advanced brain imaging”. She emphasised that being able to identify these changes sooner could be “incredibly valuable” because it would open up opportunities for people to take part in trials of new treatments, and for those with early-stage disease to benefit from disease-modifying therapies that are already being developed.
Dr Hanley also highlighted the current challenge: “Many people wait far too long for a dementia diagnosis, meaning they often miss out on these opportunities.” She noted that blood tests are less invasive, scalable, and potentially more accessible than PET scans or lumbar punctures, and that “accurate blood tests could improve how a diagnosis is made”.
While acknowledging that detailed brain imaging remains important for understanding the extent and stage of the disease, Dr Hanley said the tau PET scan study suggests “newer methods might detect Alzheimer’s-related changes earlier than existing techniques, which could enable earlier identification of disease and more precise selection of participants for clinical trials”.
Professor Jonathan Schott of Alzheimer’s Research UK pointed out that “timely diagnosis will be key to ensuring advances reach those who need them most, especially as new treatments emerge”.
Professor Roslyn Bill of Aston University added that earlier and more sensitive detection of tau pathology could improve patient selection for clinical trials and facilitate more targeted therapeutic intervention. Identifying individuals at risk, she said, “could enable interventions aimed at slowing disease progression before significant cognitive decline occurs”.
To turn these research findings into routine practice, a multi-million-pound initiative called the Blood Biomarker Challenge has been launched by the Alzheimer’s Society, Alzheimer’s Research UK, and the National Institute for Health and Care Research (NIHR). Its goal is to make blood tests for dementia diagnosis available on the NHS by 2029. As part of that effort, the ADAPT trial is investigating whether measuring p-tau217 in blood can improve diagnostic rates and patient care across diverse groups referred to memory clinics in the UK.
Dr Hanley cautioned that, exciting as the results are, “we will need further research in larger and diverse groups of people before the approaches could be used routinely”. Blood tests are unlikely to be used alone for diagnosis, but will form part of a wider assessment. Meanwhile, experts stress that modifiable risk factors — such as physical inactivity, smoking, depression, and poor cardiovascular health — could delay or prevent up to 40% of dementia cases, and that lifestyle interventions remain a powerful complement to any diagnostics developed.