Blood test may identify Alzheimer’s symptoms before they arise

A blood test may be able to detect early brain changes associated with Alzheimer’s disease, according to new research published in the journal Neurology.

Up to two decades before people develop the characteristic memory loss and confusion of Alzheimer's disease, damaging clumps of protein start to build up in their brains. The new blood test detects these changes and has moved one step closer to clinical use.

Researchers from Washington University School of Medicine in St. Louis report 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. When blood amyloid levels are combined with two other major Alzheimer's risk factors, age and the presence of the genetic variant APOE4, people with early Alzheimer's brain changes can be identified with 94 percent accuracy, the study found.

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.

The current study involved 158 adults over age 50. All but 10 of the participants in the new study were cognitively normal, and each provided at least one blood sample and underwent the current gold standard for detecting early stages of Alzheimer’s, a 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, which is promising but not accurate enough for a clinical diagnostic test.

The findings represent another step toward a blood test to identify people on track to develop Alzheimer's before symptoms arise. The test may be more sensitive than a PET brain scan at detecting the beginnings of amyloid deposition in the brain, researchers say.

Such a test may become available at doctors' offices within a few years, but its benefits will be much greater once there are treatments to halt the disease process and forestall dementia. Clinical trials of preventive drug candidates have been hampered by the difficulty of identifying participants who have Alzheimer's brain changes but no cognitive problems. The blood test could provide a way to efficiently screen for people with early signs of disease so they can participate in clinical trials evaluating whether drugs can prevent Alzheimer's dementia, according to Randall Bateman, MD, senior author and professor of neurology.

"Right now, we screen people for clinical trials with brain scans, which is time-consuming and expensive, and enrolling participants takes years," said Bateman in a statement. "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."

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

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. But testing preventive treatments requires screening thousands of healthy people to find a study population of people with amyloid build-up and no cognitive problems, a slow and expensive process.

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. Unlike blood tests, which cost a few hundred dollars, each PET scan costs upward of $4,000. A single site can only run a few dozen PET brain scans a month, because PET scanners are primarily reserved for patient care, not research studies.

"If you want to screen an asymptomatic population for a prevention trial, you would have to screen, say, 10,000 people just to get 1,500 or 2,000 that would qualify," Bateman said. "Reducing the number of PET scans could enable us to conduct twice as many clinical trials for the same amount of time and money. It's not the $4,000 per PET scan that we're worried about. It's the millions of patients that are suffering while we don't have a treatment. If we can run these trials faster, that will get us closer to ending this disease."