A blood test that diagnoses Alzheimer's years before symptoms develop could be on the horizon.
Scientists have identified chemicals linked to mild cognitive impairment (MCI) - an early sign of the disease.
It could lead to drugs or lifestyle changes being prescribed when they are most likely to be effective.
Corresponding author Professor Bin Xu, of North Carolina Central University, said: "Our work provides a new avenue for developing diagnosis and differentiation tools for Alzheimer's."
The discovery offers hope of national screening programs. It is based on post mortem brain tissue from patients who died with and without the disease.
Specific forms of a rogue protein, known as p-tau198 and p-tau217, differentiated those with MCI from older subjects without the impairment.
They show up in plasma, a yellowish liquid that carries red and white blood cells around the body.
Currently, PET (positron emission tomography) scans are required for clinical diagnosis of dementia (ADRD) or spinal lumbar punctures. Both are costly.
Identifying biomarkers for the development of non-invasive or minimally invasive and inexpensive tests is thus an urgent and unmet need.
Significant progress has been made in the last few years, such as the development of tests for tau biomarkers with blood samples.
Prof Xu said: "However, so far there are no well-established biomarkers that are capable of robustly discriminating MCI or the first stage of Alzheimer's from cognitively normal samples."
The study found several potential candidates. In particular, p-tau198 also discriminated Alzheimer's from two other neurodegenerative diseases where tau is known to clump.
Prof Xu said: "Importantly, both p-tau 198 and p-tau217 also could differentiate brain tissue of patients with MCI - an early sign of Alzheimer's.
"They could help clinicians intervene early, as new treatments become available, before significant neurological damage occurs."
Mild cognitive impairment is a transitional stage between normal ageing and dementia, leading to minor memory lapses that sometimes become more serious.
The number of dementia cases worldwide will triple to more than 150 million by 2050. There is no cure.
One of the reasons drugs have failed to date is they are given to trial participants once the memory robbing disease has already taken hold.
Prof Xu said: "Identification of p-tau198 or other promising candidates for general MCI diagnosis may have significant impact on clinical practice for much needed early Alzheimer's biomarker development."
Dementia robs sufferers of their memory and families of loved ones well before they die.
Prevention techniques can help delay onset and severity of symptoms. They include eating plenty of fresh fruit, vegetables and fish - and getting plenty of exercise.
Alzheimer's and other forms of the disease affect over 920,000 Britons, a figure that will reach two million within three decades.
The study is published in the journal ACS Chemical Neuroscience.