Regular eye tests could be used to diagnose early-stage Alzheimer's, new research suggests.
Early trials of two different techniques show a key Alzheimer's biomarker can be identified in the retina and lens.
Both methods were able to distinguish between probable Alzheimer's patients and healthy volunteers with a high level of accuracy.
After an initial eye test, more expensive and costly procedures such as PET (positron emission tomography) scans or spinal fluid analysis would then be used to confirm the disease.
Early diagnosis of Alzheimer's is essential to developing effective treatments that do more than alleviate the condition's symptoms.
Virtually every trial of a drug designed to halt or reverse Alzheimer's progression has ended in failure because the patients taking part have already suffered too much damage to their brains, scientists believe.
Shaun Frost, from Australian science agency the Commonwealth Scientific and Industrial Research Organisation, who led one of the studies, said: "We envision this technology potentially as an initial screen that could complement what is currently used.
"If further research shows our initial findings are correct, it could potentially be delivered as part of an individual's regular eye check-up.
"The high resolution level of our images could also allow accurate monitoring of individual retinal plaques as a possible method to follow progression and response to therapy."
The eye tests exploit the fact that the eye is, in effect, an extension of the brain.
In both studies, scientists looked for signs of beta-amyloid protein, which forms clumps in the brains of Alzheimer's patients.
The Australian team used the turmeric spice ingredient curcumin as a fluorescent tag that allowed beta-amyloid to show up in the retina.
A total of 200 volunteers were first asked to take a supplement of curcumin, which binds strongly to beta-amyloid. The protein was then detected in the eye using a novel imaging system.
Levels of beta-amyloid in the retina mirrored those shown in the brain by PET imaging.
Preliminary results on 40 participants showed the test picked up every participant with Alzheimer's and correctly identified more than 80pc of those who did not.
In the other study, researchers from the US company Cognoptix Inc used an ointment to apply a fluorescent label to beta-amyloid in the lens of the eye.
Laser scanning was then able to detect the protein. In tests of 40 volunteers with and without Alzheimer's, it identified those having the disease with 85pc accuracy.
Dr Simon Ridley, of Alzheimer's Research UK, said: "It is difficult to diagnose Alzheimer's accurately and, in many cases, by the time the symptoms have developed damage has already been going on in the brain.
"This research is promising, but is in the early stages and involves very small sample sizes."