New blood tests could help doctors diagnose Alzheimer’s disease faster and more accurately, researchers report — but some appear to work far better than others.

It’s tricky to tell if memory problems are caused by Alzheimer’s. That requires confirming one of the disease’s hallmark signs — buildup of a sticky protein called beta-amyloid — with a hard-to-get brain scan or uncomfortable spinal tap. Many patients instead are diagnosed based on symptoms and cognitive exams.

Labs have begun offering a variety of tests that can detect certain signs of Alzheimer’s in blood. Scientists are excited by their potential but the tests aren’t widely used yet because there’s little data to guide doctors about which kind to order and when.

“What tests can we trust?” asked Dr. Suzanne Schindler, a neurologist at Washington University in St. Louis.

While some are very accurate, “other tests are not much better than a flip of a coin.”

Demand for earlier diagnosis is increasing

More than 6 million people in the United States and millions more around the world have Alzheimer’s, the most common form of dementia. Its telltale “biomarkers” are brain-clogging amyloid plaques and abnormal tau protein that leads to neuron-killing tangles.

New drugs, Leqembi and Kisunla, can modestly slow worsening symptoms by removing gunky amyloid from the brain. But they only work in the earliest stages of Alzheimer’s and proving patients qualify in time can be difficult. Measuring amyloid in spinal fluid is invasive. A special PET scan to spot plaques is costly and getting an appointment can take months.

Even specialists can struggle to tell if Alzheimer’s or something else is to blame for a patient’s symptoms.

“I have patients not infrequently who I am convinced have Alzheimer’s disease and I do testing and it’s negative,” Schindler said.

New study suggests blood tests can be simpler

Blood tests so far have been used mostly in carefully controlled research settings. But a new study of about 1,200 patients in Sweden shows they also can work in the real-world bustle of doctors’ offices — especially primary care doctors who see far more people with memory problems than specialists but have fewer tools to evaluate them.

In the study, patients who visited either a primary care doctor or a specialist for memory complaints got an initial diagnosis using traditional exams, gave blood for testing and were sent for a confirmatory spinal tap or brain scan.

Blood testing was far more accurate, Lund University researchers reported Sunday at the Alzheimer’s Association International Conference in Philadelphia. The primary care doctors’ initial diagnosis was 61% accurate and the specialists’ 73% — but the blood test was 91% accurate, according to the findings.

Which blood tests for Alzheimer’s work best?

There’s almost “a wild West” in the variety being offered, said Dr. John Hsiao of the National Institute on Aging. They measure different biomarkers, in different ways.

Doctors and researchers should only use blood tests proven to have a greater than 90% accuracy rate, said Alzheimer’s Association chief science officer Maria Carrillo.

Today’s tests most likely to meet that benchmark measure what’s called p-tau217.

Schindler led an unusual direct comparison of several kinds of blood tests that came to the same conclusion. That type of test measures a form of tau that correlates with how much plaque buildup someone has. A high level signals a strong likelihood the person has Alzheimer’s while a low level indicates that’s probably not the cause of memory loss.