A simple blood test that measures the hallmark proteins associated with Alzheimer’s could detect the disease decades before symptoms appear, a study suggests.

The findings showed that Alzheimer’s may be present in midlife and already linked to cognitive differences, researchers said.

While more research is needed, experts said using blood tests to pinpoint changes in the brain earlier “could be incredibly valuable”.

Alzheimer’s happens when the proteins amyloid and tau abnormally build up in the brain.

For the study, researchers measured the levels of two amyloid biomarkers, as well as p-tau217, in the blood of 1,350 people without dementia in the US.

The average age of the patients was 61.

Value of plasma biomarkers

Analysis found high levels of the biomarkers in 86 patients, which were associated with worse cognitive performance, accelerated decline of verbal memory and slower processing speed in tests taken five years apart.

Researchers said the results, published in the Lancet, build on previous studies with older adults by “showing that evidence of Alzheimer’s disease neuropathology is present in midlife, although infrequent, and is already linked to measurable cognitive differences”.

“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,” they added.

“Identifying individuals with early Alzheimer’s disease neuropathology through accessible, blood-based assays could help target preventive strategies and clinical trials aimed at delaying or preventing the onset of dementia, with implications for both clinical practice and public health policy.”

Brain scans

Elsewhere, a separate study also published in the Lancet suggests a new way of performing brain scans could detect tau protein tangles before symptoms appear.

Researchers compared Flortaucipir, a radioactive tracer used in positron emission tomography (PET) scans, to a newer agent called MK6240.

The study of 682 patients in the US and Canada found MK6240 identified more than twice as many tau-positive cases as Flortaucipir in early tau regions.

Flortaucipir, also known as Tauvid, is licensed in the UK but not routinely used on the NHS.

It is estimated that about one million people in the UK have dementia, with the figure expected to rise to 1.4 million by 2040.

Alzheimer’s – the most common form of dementia – makes up between 60 per cent and 80 per cent of cases.