Blood test rivals brain scans for Alzheimer's detection, may enable early treatment

For decades, Alzheimer's disease remained a diagnosis made in hindsight. A person would begin forgetting names, losing their way home, struggling with the simplest tasks—and only then would doctors run the tests: a spinal tap to sample cerebrospinal fluid, or a PET scan to peer inside the skull. By then, the disease had already been rewriting the brain for years, sometimes decades. Now researchers at Washington University School of Medicine and Lund University in Sweden have demonstrated that a simple blood test can do what those invasive procedures do, and do it just as well.

The test measures a protein called phosphorylated tau-217, or ptau-217, which accumulates in the blood when amyloid plaques—the hallmark of Alzheimer's pathology—build up in the brain. In a study published in Nature Medicine in February 2024, researchers compared the blood test against three FDA-approved cerebrospinal fluid tests and PET brain scans, the current gold standard. They used samples from nearly 1,800 people across two cohorts: one from Sweden's BioFINDER-2 study and one from Washington University's Knight Alzheimer Disease Research Center. The results were striking. The blood test achieved 95 to 97 percent accuracy in identifying people with amyloid buildup—matching the cerebrospinal fluid tests exactly. In measuring tau tangles, another marker of neurodegeneration, the blood test actually outperformed the spinal tap tests, reaching 95 to 98 percent accuracy.

What makes this finding urgent is timing. For the first time in Alzheimer's history, drugs that can slow the disease's progression have become available to patients. More are in development. But these treatments appear to work best when started early—ideally before symptoms appear at all. The problem has always been detection. Doctors have never screened asymptomatic people for Alzheimer's because the diagnostic methods were too invasive, too expensive, too impractical for routine screening. A blood test changes that calculus entirely.

Alzheimer's disease has a long hidden phase, sometimes lasting twenty years or more, during which amyloid silently accumulates in the brain before neurons begin to die and memory loss sets in. During this presymptomatic window, the disease is theoretically most treatable. A subgroup analysis in the study showed that the ptau-217 blood test could identify asymptomatic people who already harbored amyloid plaques in their brains—people at high risk of developing cognitive impairment within the next few years. This opens a door that has never existed before: the possibility of finding and treating Alzheimer's disease before a single symptom appears.

The research team, led by Randall Bateman and Nicolas Barthélemy at Washington University, built on earlier work. Bateman's group had already created the first FDA-approved blood test for amyloid, which received a "Breakthrough Device" designation in 2019. The ptau-217 test represents the next generation. A combination of both measures is now marketed by the Washington University startup C2N Diagnostics under the name PrecivityAD2. Oskar Hansson, a neurologist at Lund University and co-senior author of the study, predicted that blood tests will soon replace cerebrospinal fluid analysis and PET imaging in specialist memory clinics. The next question, he noted, is whether the test works in primary care—whether a person's regular doctor could order it as easily as a cholesterol panel. That investigation is already underway in Sweden.

The implications are profound but still theoretical. Imagine a 55-year-old with a family history of Alzheimer's or a genetic risk variant. With a simple blood draw, they could learn whether amyloid is accumulating in their brain. If it is, they could begin treatment years before any cognitive decline, potentially preventing dementia altogether. A major clinical trial called AHEAD 3-45, launched in 2020 and ongoing at multiple sites including Washington University, is testing exactly this scenario: whether treating amyloid-positive people before symptoms emerge can prevent cognitive decline. The blood test is being used to screen participants for that trial.

But the researchers are careful about the timeline. Barthélemy noted that while therapies now exist, they do not reverse neuronal loss that has already occurred. The real promise lies in intervening earlier, in that long presymptomatic phase when amyloid is building but neurons are still intact. "We're still a few years away from such an approach," Barthélemy said, "but I think that's the future of Alzheimer's care." That future depends on one thing: the ability to find the disease before it announces itself. For the first time, a blood test makes that possible.