Samples of fluid were taken from the spine for analysis
Scientists have pinpointed compounds in the spinal fluid which may give an early warning of how fast patients with mild dementia will decline.
The level of these chemicals seems to correlate with the rate at which thinking, learning and memory skills fade as dementia takes its toll.
The finding raises hope of being able to target treatment at those in most need from an early stage.
The US study appears in the journal Archives of Neurology.
Research is currently focused on trying to find effective treatments for dementia in its early stages.
However, to test the effectiveness of new approaches researchers need to enrol people into clinical trials when they are still at the earliest stages of the disease.
But this can prove problematic, as it is difficult to anticipate how symptoms of dementia will progress in a patient still in the earliest stages.
This latest finding, from the Washington University School of Medicine, St. Louis, may provide researchers with a way of selecting patients whose condition is most likely to deteriorate, and thus from who they have most to learn when testing the impact of new therapies.
Ultimately, it might also assist doctors in deciding who to target as more invasive and potentially harmful disease-modifying treatments for dementia become available.
The latest study was based on 49 people who had been diagnosed with very mild Alzheimer's disease.
A sample of spinal fluid was taken from each, and analysed for levels of several chemicals - or biomarkers - associated with Alzheimer's.
The patients were then followed up an average of three-and-a-half years later.
The researchers linked accelerated progress of Alzheimer's to several low levels of a protein called amyloid, and high levels of two other proteins, called tau and phosphorylated tau 181.
The disease also progressed more rapidly in people whose tau level was relatively high in comparison to their amyloid level.
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said: "This interesting research could lead to a new way of detecting people with dementia early, before they develop devastating symptoms.
"This is absolutely vital if we are to find drugs that help people at an early stage.
"Everybody experiences dementia differently. This study could also help identify people at particular risk of a steep decline in thinking, learning and memory skills, making it easier to see how effective potential new treatments are."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, agreed the technique could potentially lead to earlier and more accurate diagnosis of Alzheimer's, and help scientists develop new treatments.
She said: "Although examining spinal fluids can be uncomfortable for patients - and new developments could alleviate such discomfort - this research will help scientists assess the effectiveness of treatments being tested and developed."