A blood test for Alzheimer’s disease – capable of spotting the condition years before memory loss and confusion occur – is on the horizon.

Scientists found that tiny fragments of a protein which appears in the brain of people with dementia, also starts to circulate in the blood early on in the illness.

The discovery opens the door to widespread screening for Alzheimer’s, and could enable doctors to diagnose the condition far earlier, when it may be possible to halt or reverse deterioration in the brain.

It will also be able to tell someone that they are unlikely to get the disease in the near future if they have been worried about memory loss and speed up trials into new drugs.

Dr Rosa Sancho, Head of Research at Alzheimer’s Research UK said: “A reliable blood test for Alzheimer’s disease would be a huge boost for dementia research, allowing scientists to test treatments at a much earlier stage which in turn could lead to a breakthrough for those living with dementia.”

Although there are currently no drugs or treatments, many experts now believe that preventing the onset of symptoms may be possible even if reversal of the disease is not.

Alzheimer’s disease is linked to a rise in levels of are unique structures in the brain tissue known as amyloid plaques and tau tangles which stop cells communicating.

However, the exact role of these structures is not known, and it remains unclear if they cause the disease or if they are a byproduct.

Previously, it was only possible to detect their presence by using brain scans which can be expensive, or a spinal tap which is an invasive procedure.

The level of tau in spinal fluid was already known to predict the onset of cognitive symptoms but scientists were unsure if it would be seen in the blood. 

Researchers at the Washington University School of Medicine in St Louis, developed a technique to detect minute amounts of tau in just four millilitres of blood and found that it correlated to dementia levels. Healthy people had very small levels of tau.

The research, which was presented virtually at the Alzheimer’s Association International Conference (AAIC) was hailed as a major breakthrough by British experts.

Professor Clive Ballard, professor of age-related disease, at the University of Exeter Medical School, said: “This research represents an exciting step towards developing a blood test that could help identify Alzheimer’s disease by focusing on specific sub-types of tau, one of the key proteins that becomes abnormal as part of the Alzheimer’s disease changes in the brain.  

“A lot of work will be needed to achieve standardisation of the test across laboratories – so it could still be at least five years before we see an accurate blood biomarker test for dementia in the clinic.”

Professor Nick Fox, professor of clinical neurology at University College London (UCL) added: “I think we are now seeing convincing evidence that blood tests really can identify Alzheimer’s disease with high sensitivity – and I believe we will see these entering clinical practice rapidly.”

However some experts raised concerns about the ethical implications of informing someone they were likely to develop Alzheimer’s years before symptoms emerged, and particularly before there was a treatment available.

Professor David Curtis, of the UCL Genetics Institute, said: “The potential implications of such predictive tests could raise some challenging issues for society.  

“Do people want to know that they will develop Alzheimer’s disease, given that there is no treatment? 

“Would people want to use this information for retirement planning or making advance arrangements for care?  There may well be some difficult ethical issues to think about.”

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