A test capable of diagnosing Alzheimer's disease years before there are any symptoms may be on the horizon.

One of the greatest challenges in treating the disease — which affects at least 5.5 million people in the U.S. — is that it's difficult to catch early. There's currently no reliable way to diagnose Alzheimer's until symptoms like memory loss are already recognizable. And by that time the brain has suffered years, if not decades, of damage. That's likely why many recent promising drug trials have failed.

But researchers from Washington University School of Medicine in St. Louis now say they can use a combination of a blood test, a genetic test, and a patient's age to predict with 94 percent accuracy whether or not a person will develop brain changes that are linked to Alzheimer's disease.

The blood test compares the levels of two versions of the amyloid beta protein, one of two main proteins involved in the Alzheimer’s pathology.

“We look at one form that's 40 amino acids long, and one that's slightly longer — 42 amino acids,” explained Suzanne Schindler, an assistant professor of neurology at Washington University in St. Louis and lead author on the new study. “What we found is that the ratio of those two proteins is very accurate in predicting whether someone has early Alzheimer's disease brain changes.”

The ratio, Schindler said, is roughly 88 percent accurate. To get to 94 percent accuracy, Schindler and her colleagues added in other criteria: A genetic mutation known as ApoE e4, and the person's age. This test, she added, is even more sensitive to early brain changes than the current gold standard in diagnosing Alzheimer's, a specialized brain scan known as amyloid PET.

But this testing isn't for Alzheimer’s disease itself. Some people develop amyloid plaques and never get dementia.

“This test is really for the amyloid brain changes, not for clinical symptoms of Alzheimer's disease,” Schindler emphasized.

Still, Schindler says the test could be used within a year or two to screen candidates for Alzheimer’s drug trials. In fact, she says it could be a “game changer.”

“We think that by using this blood test, that we could recruit people much more quickly ... and that would really translate to Alzheimer's disease drug trials being cheaper and faster,” she said. “And hopefully that will mean that we can find a drug that works.”

In fact, Schindler says it’s quite possible that drugs that have failed in previous clinical trials might work if given to patients diagnosed earlier.

“A lot of the drug trials that have been done so far have used patients who have significant symptoms and are pretty far along in the course of the disease,” she explained. “It's much harder to reverse the disease than to prevent it. So, we really need to find a way to figure out who's at risk and enroll them in studies before they even have symptoms.”

Schindler says this test could make a big difference for patients whose dementia doesn’t have a clear-cut cause. Confirming — or refuting — the presence of early amyloid brain changes could guide patients’ and doctors’ decisions about which treatments to pursue.

Eventually, Schindler would like to see a test for amyloid brain changes become as widespread and routine as cholesterol testing. But she says it’s too soon for that.

“We don't have effective treatments right now,” Schindler said. “So, if we test someone and they're positive, and they don't have clinical symptoms, there's not necessarily a lot they can do about it.”

Which, Schindler says, leads back to the argument for accelerating the development and testing of Alzheimer’s drugs.