What a new blood test means for Alzheimer’s patients

A new blood test could mean an easier and less expensive way to help diagnose Alzheimer’s disease in patients with symptoms, potentially resulting in earlier treatment.

Alzheimer’s disease is underdiagnosed and accurate blood tests “may revolutionize the accuracy of diagnosis and provide a cleaner, quicker path to research participation and treatment,” according to a press release from the Alzheimer’s Association.

The new blood test was first discovered by Swedish researchers. According to a paper published July 28, 2024, in JAMA, a study of 1,213 patients in Sweden who were being evaluated for their cognitive symptoms had blood samples tested for forms of two proteins that accumulate in the brains of people with Alzheimer’s disease: p-tau17 and amyloid beta. These proteins can interfere with brain function by forming amyloid plaques between neurons and tau tangles inside of neurons, according to the National Institute of Aging.

The blood test results were compared to spinal fluid analysis for the same biomarkers to determine accuracy. Correct diagnosis of Alzheimer’s disease by Swedish dementia experts using clinical examination increased from 73 percent to 91 percent when adding the blood test.

The Swedish study found the blood test’s accuracy ranged from 88 to 92 percent. It also found blood levels of just p-tau17 – not considered in combination with amyloid beta levels – predicted Alzheimer’s disease with a 90 percent accuracy.

The blood test, called PrecivityAD2, can be purchased in the United States, but has not been approved by the federal Food and Drug Administration, nor do most health insurers cover it, according to an Aug. 13, 2024, National Institutes of Health article.

Two other tests now exist to help diagnose Alzheimer’s disease: an analysis of spinal fluid or a PET (positron emission tomography) brain scan. However, these tests may not be fully covered by a patient’s health insurance or available everywhere, according to Sean Horrigan, DO, FAAN, medical director of Neurologists of Cape Cod, in Hyannis. He said most Alzheimer’s cases are diagnosed by a neurologist without using a spinal fluid test or PET scan.

“I would order these tests if I have someone with mild symptoms who might be a candidate” for drug treatment, he added.

Test Part of Recent Developments

“When I was in training, diagnosis (of Alzheimer’s disease) was made on a clinical basis,” using an exam and patient history, “roughly 80 percent of the time,” Dr. Horrigan said. “We usually got the diagnosis right.”

The new blood test, as well as PET scans for amyloid beta plaques in the brain, and spinal fluid tests, do not conclusively prove a diagnosis, as patients can have elevated levels of these biomarkers and not have symptoms, Dr. Horrigan said. The tests can show if a patient is at increased risk of developing Alzheimer’s disease or help determine if the cause of cognitive symptoms is Alzheimer’s or another type of dementia or health condition. Diagnosing dementia can be complex, as it often coexists with one or more other conditions, such as cardiovascular disease, high cholesterol levels or diabetes, he said.

“I would not suggest testing for patients with no symptoms,” even if they have a family history of Alzheimer’s disease, Dr. Horrigan said.

“The blood test is very sensitive for people with symptoms,” he said, and may help identify patients who might benefit from medical treatment.

“In the past two years, two anti-amyloid drugs (lecanemab and donanemab) became available,” Dr. Horrigan said. “These medicines can’t cure Alzheimer’s disease,” or stop progression, but they can slow down progression. They’re appropriate for people with early stages of the disease, not severe dementia.

“This is a fast-changing landscape that we’re in,” he said.

Ordering and Interpreting Tests

The Swedish study also found the accuracy of primary care physicians diagnosing Alzheimer’s disease rose from 61 percent when using a clinical exam, cognitive testing and a PET scan, to 91 percent when adding the blood test.

Dr. Horrigan said primary care doctors in the United States typically would not order a spinal fluid test or a PET scan but refer patients to a neurologist.

“These tests are best utilized by a person specializing in neurological care,” he said, adding that he would only order one if “it could change how I take care of that person.”

The PrecivityAD2 test is produced by C2N Diagnostics, which also markets two other blood tests for indicators of Alzheimer’s disease: Precivity AD and Precivity-ApoE. The first checks for amyloid beta ratio and ApoE (apolipoprotein E), a marker for genetic risk of Alzheimer’s disease. The second is simply the genetic test for ApoE. The company’s website says these tests are intended for physicians to order and being new, are not covered by private insurance, Medicare or Medicaid, but some insurers and Medicare Advantage plans may cover some or all of the costs on an individual basis. Results are sent to the doctor who ordered them.

Dr. Horrigan said he would not favor having such tests being made available directly to patients, as interpreting the results and what they mean to the affected person and their family should be a discussion between a doctor and patient.

"For instance, with a genetic test, extensive counseling is often required to guide patients through the implications of their results," he said.