When a patient complains of forgetfulness, a neurologist might not know immediately whether it results from normal aging, reduced blood flow to the brain—or, more ominously, Alzheimer’s disease. For much of the past century, a definitive Alzheimer’s diagnosis could only be made during an autopsy. Brain imaging and spinal fluid tests now make it possible to spot the disease in patients even before the initial symptoms appear. But these invasive tests are expensive and generally limited to research settings that are not part of routine care for the millions of people suffering from the most common neurodegenerative disorder.

An era in which an Alzheimer’s diagnosis can begin in a doctor’s office is now arriving. Advances in technologies to detect early signs of disease from a blood sample are helping doctors to identify the memory-robbing disorder more accurately and to screen participants more quickly for trials of potential treatments for the more than five million people in the U.S. afflicted with Alzheimer’s. (Estimates predict that, by 2030, there will be 76 million people worldwide who will receive a diagnosis of Alzheimer’s or other dementias.)

Last fall, a blood test developed by C2N Diagnostics in St. Louis, Mo., became available to most of the U.S. as a routine lab test—regulated under the CMS Clinical Laboratory Improvement Amendments (CLIA) program. It has also received a CE mark as a diagnostic medical device in the European Union—indicating it has met safety, health and environmental protection standards for the region.

“The development of a blood-based test for Alzheimer’s disease is just phenomenal,” says Michelle Mielke, a neuroscientist and epidemiologist at the Mayo Clinic. “The field has been thinking about this for a very long time. It’s really been in the last couple of years that the possibility has come to fruition.”

The C2N test, called PrecivityAD, uses an analytic technique known as mass spectrometry to detect specific types of beta-amyloid, a protein fragment that is a pathological hallmark of disease. Beta-amyloid proteins accumulate and form plaques visible on brain scans two decades before a patient notices memory problems. As plaques build up in the brain, levels of beta-amyloid decline in the surrounding fluid. Such changes can be measured in spinal fluid samples—and now in blood, where beta-amyloid concentrations are significantly lower. PrecivityAD is the first blood test for Alzheimer’s to be cleared for widespread use and one of a new generation of such assays that could enable early detection of the leading neurodegenerative disease—perhaps decades before the onset of the first symptoms.

PrecivityAD is meant for 60- to 91-year-olds with early signs of cognitive impairment. The prescribing physician ships patient blood samples for analysis at C2N’s lab and receives results within 10 business days. The results—a probability score that reflects the likelihood of an amyloid-positive brain scan—are calculated using a proprietary algorithm that incorporates the person’s age with measurements of beta-amyloid and a protein called apolipoprotein E that is known to influence Alzheimer’s disease risk.

Rather than serving as a stand-alone tool, the results are meant to enhance the accuracy of a clinical diagnosis by distinguishing Alzheimer’s dementia from memory loss caused by other conditions. The test costs $1,250 and is not currently covered by insurance, though a financial assistance program can bring out-of-pocket costs down to between $25 and $400 for eligible patients, says C2N’s chief executive Joel Braunstein.

By comparison, beta-amyloid tests using positron-emission tomography (PET) brain imaging typically cost around $5,000 and are typically not covered by insurance, and those that sample cerebrospinal fluid (CSF) usually cost from $800 to $1,000. Compared with these more invasive and burdensome procedures, the ease and lower cost of blood tests open up many exciting possibilities for clinical use and therapeutic development,” says Adam Boxer, a neurologist at the University of California, San Francisco. “Blood tests can be collected from people repeatedly in remote locations or in their homes.” No drugs have yet been approved that change the course of Alzheimer’s. But readily available early tests could improve treatment by letting patients take measures to stay healthy, affording them an opportunity to plan for an uncertain future and participate in clinical trials.

From a preventive standpoint, blood tests could “help identify who’s at risk,” Mielke says. Testing could also be used to screen potential participants for experimental drugs. In some past trials of beta-amyloid-reducing treatments, 15 to 30 percent of patients who met clinical criteria for Alzheimer’s turned out not to have brain amyloid. Nowadays trials often require participants to show evidence of disease pathology through PET scans or CSF measures. Prescreening with a cheap blood test could halve the number of PET scans needed to enroll volunteers, according to a new study published on January 22 in the journal Brain.

This would lower the cost of trials, which means “more potential treatments can be tested, and that increases the chances of finding a cure,” says Elisabeth Thijssen, a researcher studying blood biomarkers for Alzheimer’s at Amsterdam University Medical Centers in the Netherlands. Blood tests would be particularly helpful in identifying patients for trials of potential drugs that could be most effective long before the first symptom of cognitive decline.

Looking for beta-amyloid is not the only option. Some researchers believe other disease markers—for example, certain forms of the protein tau—could prove more promising when incorporated in blood tests for Alzheimer’s. Beta-amyloid levels start to drop very early in the disease process and then reach a plateau, whereas tau markers go up later and continue to rise. That observation suggests amyloid tests could work better for early detection while tau levels are more meaningful at later stages of the disease, when someone is on the verge of decline or already symptomatic, says Oskar Hansson, a neurologist at Lund University in Sweden. Last year Thijssen and Hansson published separate studies showing that tau blood tests could distinguish Alzheimer’s from other neurodegenerative diseases nearly as well as CSF measurements and PET scans. Quanterix, a company in Billerica, Mass., has developed an immunoassay that detects amyloid and tau in conjunction with other neurological markers and inflammatory proteins. So far these tests are not available outside of research settings.

“We researchers are super enthusiastic” about these tests, Thijssen says. Most studies have been conducted in extensively studied groups of patients in neurology clinics, however. “Now we have to make the step into the real world,” she says. When a new patient comes in with memory complaints, “is a blood test going to help physicians make a proper diagnosis?”

Patients in other settings may have other ailments that could affect the accuracy of assays. Some medical conditions can influence the levels of blood proteins, possibly skewing test results. “If somebody has chronic kidney disease, that can affect the clearance of proteins,” Mielke says. “Individuals with a high body mass index tend to have higher blood volume, so that could reduce protein levels.”

UCSF neurologist Gil Rabinovici agrees that “all these markers need to be validated in more diverse and generalizable cohorts.” He is helping to lead a new study that will test blood assays against amyloid PET scans in 5,000 patients recruited at 350 clinical sites—with an emphasis on patients from Black and Latinx populations, which are historically underrepresented in dementia research.