CHICAGO, Aug. 1 (Xinhua) -- Researchers from Washington University School of Medicine in St. Louis reported that they can measure levels of the Alzheimer's protein amyloid beta in the blood and use such levels to predict whether the protein has accumulated in the brain.

The test uses a technique called mass spectrometry to precisely measure the amounts of two forms of amyloid beta in the blood: amyloid beta 42 and amyloid beta 40. The ratio of the two forms goes down as the amount of amyloid beta deposits in the brain goes up, according to a report published in the journal Neurology on Thursday.

The study involved 158 adults aged over 50. All but 10 of the participants in the new study were cognitively normal, and each provided at least one blood sample and underwent one PET brain scan.

The researchers classified each blood sample and PET scan as amyloid positive or negative, and found that the blood test from each participant agreed with his or her PET scan 88 percent of the time.

In an effort to improve the test's accuracy, the researchers incorporated several major risk factors for Alzheimer's. Age is the largest known risk factor; after age 65, the chance of developing the disease doubles every five years.

A genetic variant called APOE4 raises the risk of developing Alzheimer's three- to fivefold. And gender also plays a role: Two out of three Alzheimer's patients are women.

With these risk factors included, the researchers found that age and APOE4 status raised the accuracy of the blood test to 94 percent. Sex did not significantly affected the analysis.

Further, the results of some people's blood tests initially were considered false positives because the blood test was positive for amyloid beta but the brain scan came back negative.

But some people with mismatched results tested positive on subsequent brain scans taken an average of four years later. The finding suggests that, far from being wrong, the initial blood tests had flagged early signs of disease missed by the gold-standard brain scan.

As part of the study, the researchers analyzed the enrollment process for a prominent Alzheimer's prevention trial called the A4 study that used PET scans to confirm the presence of early Alzheimer's brain changes in potential participants.

They concluded that prescreening with a blood test followed by a PET scan for confirmation would have reduced the number of PET scans needed by two thirds.

"Right now we screen people for clinical trials with brain scans, which is time-consuming and expensive, and enrolling participants takes years,"said senior author Randall J. Bateman, professor of neurology.

"But with a blood test, we could potentially screen thousands of people a month. 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, "Bateman added.

There is growing consensus among neurologists that Alzheimer's treatment needs to begin as early as possible, ideally before any cognitive symptoms arise. By the time people become forgetful, their brains are so severely damaged no therapy is likely to fully heal them.