By Robert Preidt
But it's possible to measure levels of amyloid beta in the blood and use that information to determine whether the protein has accumulated in the brain, they added.
Combining blood amyloid levels with two other major Alzheimer's risk factors -- age and the genetic variant APOE4 -- can identify people who have early Alzheimer's brain changes with 94% accuracy, according to the scientists from Washington University School of Medicine in St. Louis.
The study included 150 adults over age 50 who had no thinking or memory problems.
The blood test may be even more sensitive than the current gold standard -- a PET brain scan -- at detecting early amyloid accumulation in the brain, according to the authors.
The findings advance efforts to have a blood test to identify people who will develop Alzheimer's before they have symptoms, and such a test could be available in doctors' offices within a few years, the researchers said.
They added that the benefits of the blood test would be even greater once there are treatments to stop the progress of Alzheimer's disease.
The researchers also noted that one difficulty in clinical trials of Alzheimer's drugs is identifying patients who have Alzheimer's brain changes but no symptoms. The blood test could provide an efficient way to find people with early signs of the disease to participate in drug clinical trials.
"Right now, we screen people for clinical trials with brain scans, which is time-consuming and expensive, and enrolling participants takes years," explained study senior author Dr. Randall Bateman, a professor of neurology.
"But with a blood test, we could potentially screen thousands of people a month," he said in a Washington University news release. "That means we can more efficiently enroll participants in clinical trials, which will help us find treatments faster, and could have an enormous impact on the cost of the disease as well as the human suffering that goes with it."
Maria Carrillo, chief science officer the Alzheimer's Association, said such a test would be welcomed.
"There is a great need for simple, reliable, inexpensive, noninvasive and easily available tools to support early detection and accurate diagnosis of Alzheimer's," she said.
"That said, while the results are encouraging, none of these tests is ready for use in doctors' offices. They need to be verified in larger and more diverse populations," Carrillo added.
"In fact, rather than in doctors' offices, the first uses for these new techniques/technologies may be in clinical trials to identify possible participants who are most likely to benefit from the tested intervention," she said.
The study was published Aug. 1 in the journal Neurology.