A simple blood test that detects signs of brain damage could be used to spot developing Alzheimer's disease, say scientists.
The test looks for a structural protein that leaks out of damaged or dying neurons.
In a population of 247 people with a genetic mutation known to trigger young-onset Alzheimer's, the test revealed higher than normal levels of the protein that rose over time.
Protein levels were low and remained largely steady in 162 unaffected relatives who had inherited a healthy form of the gene.
Dr Brian Gordon, from Washington University School of Medicine in the US, said: "This is something that would be easy to incorporate into a screening test in a neurology clinic.
"We validated it in people with Alzheimer's disease because we know their brains undergo lots of neurodegeneration, but this marker isn't specific for Alzheimer's.
"High levels could be a sign of many different neurological diseases and injuries."
The test could also be used to identify people with brain damage caused by multiple sclerosis (MS), stroke, or traumatic injury, said the researchers whose findings appear in the journal Nature Medicine.
Patients taking part in the trial were recruited by the Dominantly Inherited Alzheimer's Network (Dian), an international consortium investigating the root causes of Alzheimer's disease.
They were all from families with rare genetic variants that cause Alzheimer's in people in their 50s, 40s or even 30s.
A parent with one of the mutations has a 50 per cent chance of passing the devastating genetic fault on to their child.
Another part of the study found that people with rapidly rising blood levels of the test protein were more likely to perform poorly in mental ability tests.
Scans revealed they were also most likely to have experienced brain shrinkage.
All kinds of neurological damage can cause the molecule, known as neurofilament light protein, to spill out of neurons into the blood, said the scientists.
Before the test can be put to practical use, researchers will need to determine how much of the protein in the blood should be considered abnormal, and how fast it can rise before becoming a cause for concern.
Dr Gordon said: "I could see this being used in the clinic in a few years to identify signs of brain damage in individual patients.
"We're not at the point we can tell people 'in five years you'll have dementia'. We are all working towards that."
Dr Rosa Sancho, head of science at the charity Alzheimer's Research UK, said: "We know that the brain changes associated with Alzheimer's begin well over a decade before memory problems start.
"This presents a key window of opportunity for tests that could detect the disease at the earliest stages and help to bring diagnosis forward by many years."
