A blood test could detect a range of brain disorders before serious symptoms strike, research suggests.
Dementia – an umbrella term for a loss of brain function, with Alzheimer's being the most common form of the disease – has no set diagnostic test, with medics assessing suspected patients via memory analyses, mental-agility evaluations and brain scans.
While the memory-robbing disease is considered incurable, treatments can temporarily ease symptoms, with these being more effective when administered early in dementia's onset.
The concept of a blood test to detect brain disorders is not new, with past studies suggesting biomarkers circulate in a patient's blood before they develop symptoms of a particular disease.
After analysing the blood samples of more than 3,000 people, scientists from King's College London found a single biomarker could pick up on diseases like dementia in people with mild "cognitive issues" with up to 100% accuracy.
Detecting conditions "when clinical symptoms are not definitive" could help to avoid misdiagnosis, with dementia often being confused for depression in its early stages.
"For the first time we have shown across a number of disorders that a single biomarker can indicate the presence of underlying neurodegeneration with excellent accuracy," said study author Dr Abdul Hye.
"Though it is not specific for any one disorder, it could help in services such as memory clinics as a rapid screening tool to identify whether memory, thinking or psychiatric problems are a result of neurodegeneration."
Neurodegeneration is another umbrella term for a range of diseases caused by a progressive loss of nerve cells in the brain, affecting a person's memory, attention span and thinking skills.
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Biomarkers can be taken from the cerebrospinal fluid that surrounds the brain and spine, but this has to be extracted via an invasive procedure.
Neurodegenerative diseases tend to cause damage to a patient's nerve fibres, resulting in the release of so-called neurofilament light chain (NfL), a protein that can then be detected at low levels in the blood.
"For neurodegenerative diseases like Alzheimer's, Parkinson's or motor neurone disease, a blood test to allow early diagnosis and help us monitor disease progression and response to treatment would be very helpful," said co-author Professor Ammar Al-Chalab.
"Neurofilament light chain is a promising biomarker that could speed diagnosis of neurodegenerative diseases and shorten clinical trials."
The scientists analysed the blood samples of more than 3,000 individuals, some of whom had been diagnosed with a neurodegenerative disease.
Results, published in the journal Nature Communications, reveal NfL levels were higher among those with a disorder compared with the healthy controls.
The levels peaked in the participants with Down syndrome dementia, motor neurone disease and frontotemporal dementia – a less common form of the disease that affects behaviour and language.
"This study shows neurofilament light chain levels were particularly increased in adults with Down syndrome, who have a genetic predisposition for Alzheimer's disease," said co-author Professor Andre Strydom.
"Furthermore, we showed those individuals with a dementia diagnosis following onset of Alzheimer's disease had higher levels than those who did not.
"This suggests the new marker could potentially be used to improve the diagnosis of Alzheimer's in people with Down syndrome, as well as to be used as biomarker to show whether treatments are effective or not.
"It is exciting that all that could be needed is a simple blood test, which is better tolerated in Down syndrome individuals than brain scans."
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The scientists also tested the participants' NfL levels at particular age "cut-offs". Many neurodegenerative diseases become more common over time, with these cut-offs representing the age most would be diagnosed.
Among the participants over 65 years old, the test was 90% accurate at picking up on any form of neurodegeneration. This rose to 100% accuracy when detecting motor neurone disease and Down syndrome dementia specifically.
The test also distinguished between the participants with neurodegeneration and depression.
"Blood tests have great potential to improve the diagnosis of dementia both in specialised memory clinics and in primary care," said co-author Professor Oskar Hansson, from Lund University in Sweden.
"Plasma NfL can be extremely useful in a number of clinical scenarios, which can greatly inform doctors, as shown in this large study."
Similar tests are reportedly already routinely being used in Sweden and the Netherlands.
"Our age-related cut-offs can provide a benchmark and quick accessible test for clinicians, to indicate neurodegeneration in people who are exhibiting problems in thinking and memory," said Dr Hye.
Lead author Dr Nicholas Ashton agreed, adding: "We are entering an exciting period where blood tests like plasma NfL, in combination with other emerging blood biomarkers like phosphorylated tau, are starting to give us a meaningful and non-invasive insight into brain disorders."
Tau – a protein – forms tangles in the brain of Alzheimer's patients, acting as a hallmark of the disease.
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