A new diagnostic test using a few drops of blood could be used detect Alzheimer's disease years before symptoms begin to show.

By 2050 more than 170,000 New Zealanders are forecast to have dementia - the majority of which will be Alzheimer's.

Early intervention is the best way to treat the disease, but this is almost impossible as it cannot be diagnosed until symptoms such as memory loss appear.

Researchers hope that will change with the discovery of a biomarker in the blood which can predict the very early stages of Alzheimer's disease.

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The research, published in Nature, found the marker predicted early Alzheimer's to 90 per cent accuracy.

The study was carried out by Japanese and Australian scientists, who analysed blood samples from large groups in both countries to verify the results.

Kiwi scientists say the international discovery is groundbreaking and could speed up their research into treatments for Alzheimer's as well as diagnosing the disease much earlier.

One of the hallmarks of Alzheimer's disease is an abnormal peptide called beta-amyloid, which builds up in the brain. This process begins about 30 years before the onset of dementia symptoms.

The new test detects a plasma biomarker that predicts that peptide being deposited.

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The test offers a far cheaper, less invasive way to test for Alzheimer's than the brain imaging or lumbar punctures currently used, according to Dr Brigid Ryan, a research fellow at New Zealand's Centre for Brain Research specialising in dementia biomarkers.

Because those tests are expensive and invasive, most patients are diagnosed based on symptoms. But these are difficult to distinguish from other forms of dementia. A definitive clinical blood test would be a big step forward, Ryan said.

She cautioned long-term studies were still needed and a blood test for Alzheimer's in the wider population was some time away.

For Professor Sir Richard Faull, director of Brain Research New Zealand, the test's potential lies in trialling drug or lifestyle treatments for Alzheimer's.

Treatment is most effective when started early, so an early diagnostic test would be "an enormous breakthrough" for Kiwi researchers working to slow the disease's progress, he said.

Dementia expert Professor Sir Richard Faull says a new
Dementia expert Professor Sir Richard Faull says a new "breakthrough" blood test for Alzheimer's research could be a key tool for researchers at Brain Research New Zealand. File photo / Chris Gorman

The neuroscientist has set up dementia prevention research clinics, enrolling patients with very early stage Alzheimer's and dementia.

Delaying the progression of Alzheimer's disease by five years in the New Zealand population would cut its prevalence in half, Faull said.

"People would live longer, they'd be more intellectually active and able to enjoy life, and they'd probably die of something else."

Blood tests could some day warn people they are at risk and encourage lifestyle changes, he said.

"There's no sense in telling a person they're going to be at risk of getting Alzheimer's in 5-10 years unless you can say 'Here are all these things that you can do now to stop the onset and progression of the disease'."

Alzheimer's New Zealand chief executive Catherine Hall welcomed the progress made in the study, but cautioned that breakthroughs are reported "almost weekly".

"We really want to see progress on these things but there is a tension between recognising the progress made ... and not sending people with dementia and their families on a rollercoaster ride," she said.

"A simple diagnostic test for dementia would be a really powerful tool and a huge step forward, but it needs to be really reliable and my understanding is that is a long way away."

WHAT IS ALZHEIMER'S DISEASE?
• Alzheimer's is a debilitating dementia characterised by memory loss, difficulty thinking and behavioural changes. Plaque buildup in the brain is a hallmark of the disease and starts up to 30 years before symptoms appear.
• Early warning symptoms include difficulty performing normal tasks, language issues, mood changes and memory loss.
• More than 60,000 New Zealanders have dementia - the majority Alzheimer's - costing the country $1.7 billion each year. By 2050 more than 170,000 New Zealanders are forecast to have Alzheimer's.
• There is no cure - research is focused on delaying the progress of the disease.