A newly developed blood test for Alzheimer's has diagnosed the disease as accurately as methods that are far more expensive or invasive, scientists reported on Tuesday, a significant step toward a longtime 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 problems 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."

Nearly 6 million people in the United States and roughly 30 million worldwide have Alzheimer's, and their ranks are expected to more than double by 2050 as the population ages.

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.

Experts said blood tests would accelerate the search for new therapies by making it faster and cheaper to screen participants for clinical trials, a process that now often takes years and costs millions of dollars because it relies on expensive methods like PET scans of the brain and spinal taps for cerebrospinal fluid.

But the ability to diagnose Alzheimer's with a quick blood test would also intensify ethical and emotional dilemmas for people deciding whether they wanted to know they had a disease that does not yet have a cure or treatment.

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 brain scans, was as good as PET 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.

Researchers said that the results would need to be replicated in clinical trials in more populations, including those reflecting more racial and ethnic diversity. The test will also need to be refined and standardized so results can consistently be analyzed in labs, and will need approval by federal regulators.

Currently, Alzheimer's diagnoses are made mostly with clinical assessments of memory and cognitive impairment, as well as interviews with patients' family members and caregivers.

"This test really opens up the possibility of being able to use a blood test in the clinic to diagnose someone more definitely with Alzheimer's," said Maria Carrillo, chief science officer at the Alzheimer's Association. "Amazing, isn't it? I mean, really, five years ago, I would have told you it was science fiction."