A newly developed blood test for Alzheimer's has diagnosed the disease as accurately as methods that are far more expensive or invasive, scientists have reported, a significant step towards a long-time goal for patients, doctors and dementia researchers.

The test has the potential to make diagnosis simple, affordable and widely available.

The test determined whether people with dementia had Alzheimer's instead of another condition.

And it identified signs of the degenerative, deadly disease 20 years before memory and thinking problems were expected in people with a genetic mutation that causes Alzheimer's, according to research published in Jama Network Open and presented at the Alzheimer's Association International Conference.

Such a test could be available for clinical use in as little as two to three years, the researchers and other experts estimated, providing an affordable, simple way to diagnose whether people with cognitive issues were experiencing Alzheimer's, rather than another type of dementia.

A blood test like this might also eventually be used to predict whether someone with no symptoms would develop Alzheimer's.

"This blood test very, very accurately predicts who's got Alzheimer's disease in their brain, including people who seem to be normal," said Dr Michael Weiner, an Alzheimer's disease researcher at the University of California, San Francisco, who was not involved in the study.

"It's not a cure, it's not a treatment, but you can't treat the disease without being able to diagnose it. And accurate, low-cost diagnosis is really exciting, so it's a breakthrough."

Blood tests for Alzheimer's, which are being developed by several research teams, would provide some hope in a field that has experienced failure after failure in its search for ways to treat and prevent a devastating disease that robs people of their memories and ability to function independently.

The test, which measures a form of the tau protein found in tangles that spread throughout the brain in Alzheimer's, proved remarkably accurate in a study of 1,402 people from three different groups in Sweden, Colombia and the United States.

It performed better than MRI (magnetic resonance imaging) brain scans, was as good as PET (positron emission tomography) scans or spinal taps, and was nearly as accurate as the most definitive diagnostic method: autopsies that found strong evidence of Alzheimer's in people's brains after they died.

NYTIMES