A new and inexpensive blood test for Alzheimer’s disease is almost 100 per cent accurate, outperforming magnetic resonance imaging (MRI) and existing blood tests.

The test was able to discern Alzheimer’s from other forms of dementia, and in people showing no symptoms.

In persons at known genetic risk, the test “may be able to detect the disease as early as 20 years before the onset of cognitive impairment”.

These are the findings of a large international study published in the Journal of the American Medical Association (JAMA) and simultaneously presented at the Alzheimer’s Association International Conference.

The researchers describe their findings as “profound”.

Scientists not involved in the study are equally enthusiastic, describing the test as “exciting” and “amazing”.

There appears to be widespread agreement that the test is a genuine “breakthrough” – a term wildly overused in study press releases.

Why is this such a big deal?

To a large extent, diagnosis of Alzheimer’s is an educated guess, at least in living patients.

The gold diagnostic standard has relied on identifying amyloid plaques and tau tangles in the brain, usually when the patient has already died.

This is why, as the authors of the new study note: “In the global action plan against dementia, the World Health Organisation has specified improved diagnostics as a key area, since it is important for optimal disease management and treatment.”

And as The New York Times observed in its coverage of the study: “Blood tests for Alzheimer’s, which are being developed by several research teams, would provide some hope in a field that has experienced failure after failure in its search for ways to treat and prevent a devastating disease that robs people of their memories and ability to function independently.”

Indeed, unreliable diagnosis is getting in the way of finding genuinely effective treatments rather than the endless “promising” ones we hear about from week to week.

Last year, as The New Daily reported, millions of people believe they have Alzheimer’s – when instead they have a form of newly discovered dementia that mimics Alzheimer’s but is caused by a different mechanism in the brain.

One likely consequence of this confusion is that some clinical drug trials for Alzheimer’s may have been abandoned too soon – because their results were compromised.

Study participants with the Alzheimer’s lookalike may have masked what were potentially positive outcomes.

What did the study find?

The researchers found that measurements of phospho-tau217 (p-tau217), one of the tau proteins found in brain tangles, could provide “a relatively sensitive and accurate indicator of both plaques and tangles in living people”.

An indication of plaques and tangles corresponds to a diagnosis of Alzheimer’s.

The p-tau217 blood test was trialled on 1402 cognitively impaired and unimpaired research participants from studies in Arizona, Sweden and Colombia.

Dr Oskar Hansson is Professor of Clinical Memory Research at Lund University, Sweden, senior author on the study.

Dr Hansson spearheaded the international collaborative effort.

In a prepared and somewhat muted statement he said: “The p-tau217 blood test has great promise in the diagnosis, early detection and study of Alzheimer’s.

“While more work is needed to optimise the assay and test it in other people before it becomes available in the clinic, the blood test might become especially useful to improve the recognition, diagnosis, and care of people in the primary care setting.”

Dr Eric Reiman is executive director of Banner Alzheimer’s Institute in Phoenix and a senior author on the study.

He said: “Blood tests like p-tau217 have the potential to revolutionise Alzheimer’s research, treatment and prevention trials, and clinical care.”

Dr Reiman predicted that impact of the blood test “in both the research and clinical setting will become readily apparent within the next two years”.

An earlier study estimated that about 100 million people worldwide will have Alzheimer’s disease in 2050.