Doctors could soon diagnose Alzheimer's disease by carrying out tests on fluid taken from the spine.
It is sometimes difficult to accurately diagnose Alzheimer's
Scientists in the United States say the technique is more accurate than brain scans or memory tests, which are normally used to diagnose the disease.
They believe the test could also help doctors to identify patient's at risk of Alzheimer's much earlier.
This could enable these patients to receive early treatment to stop the disease from spreading.
The fluid in the spine, which is called Cerebrospinal fluid (CSF), also bathes the brain. Previous studies have shown that it contains different levels of key chemicals in patients with Alzheimer's.
This study also found that they have lower levels of beta amyloid - a chemical that can clump together to form brain-damaging plaques - and higher levels of tau - a chemical that can stop the brain's neurons from working properly.
Dr Trey Sutherland and colleagues at the US National Institute of Mental Health performed spinal taps on 136 Alzheimer's patients and 72 people without the disease.
They found that patients with the disease had less than half the normal level of beta amyloid while their tau levels were more than double those of people without the disease. This pattern was found in 90% of Alzheimer's patients involved in the study.
The researchers also analysed 51 previous studies examining the chemical make-up of spinal fluid taken from patients with the disease.
Almost all reported a similar pattern - much lower levels of beta amyloid and much higher levels of tau.
However, they also found that in some cases patients without the disease had levels of these chemicals in line with those with Alzheimer's.
As a result, they concluded that tests on spinal fluid would not be able to diagnose Alzheimer's in all patients.
But they suggested that it could still identify those at risk of developing the disease if doctors could detect changes in these key chemicals.
"Perhaps the most important future use for such biomarkers is in the prospective study of people at risk for developing Alzheimer's disease," said Dr Sunderland.
"By establishing a person's baseline and tracking levels over time, we might be able to interpret gradual changes as a sign that he or she is developing the disorder."
Writing in the Journal of the American Medical Association, the researchers said further studies were needed before the test could be used more widely to diagnose the disease.