Alzheimer’s Disease: Understanding the FDA-Approved Blood Test for Diagnosis

A blood test that is highly accurate at diagnosing Alzheimer’s disease has been approved by the Food and Drug Administration (FDA) for patients 55 and older who already have memory loss or other signs and symptoms of Alzheimer’s disease.

The test, the first of its kind to be FDA approved, measures the levels of two proteins in blood plasma: phosphorylated tau 217 (p-tau217) and beta-amyloid 1-42. In a study of 499 adults who presented with cognitive concerns, a “positive” blood test was found to be 91.7% as accurate as a PET scan or spinal fluid testing, although 20% of people had “indeterminate” results. Of those with “negative” results, 97% of the test results were accurate.  

“The blood test is one way of evaluating the same thing that spinal fluid and PET scans may see, which is determining the biological process going on in a patient who is experiencing Alzheimer’s disease symptoms, such as memory loss,” says Dr. Lawrence S. Honig, a neurologist at NewYork-Presbyterian/Columbia University Irving Medical Center.

More than 7 million people in the U.S. have Alzheimer’s disease. About one in five women and one in 10 men develop dementia (a group of medical conditions or disorders that result in mental decline, and diminished ability to think, remember and reason) due to Alzheimer’s. Alzheimer’s is partly or fully responsible for more than 75% of dementia cases.

Other than memory loss, signs and symptoms of Alzheimer’s include difficulty with reasoning and concentration, decreased judgment, difficulty completing routine tasks, and changes in personality and mood. But a key indicator of the disease is the accumulation of abnormal aggregated proteins in the brain, including beta-amyloid and tau. Abnormal tau generally appears later on, after amyloid plaques have already developed, and is also linked more closely to cognitive decline.

A previous study of 1,213 patients with cognitive symptoms found that a similar type of blood test was more accurate in diagnosing Alzheimer’s disease than providers who used only cognitive tests and imaging to come to a diagnosis. In that study conducted in Sweden, dementia specialists who used standard clinical evaluations, such as cognitive exams, but did not include PET scans or spinal fluid tests, had a diagnostic accuracy of 73%, while primary care doctors who used the same approach were 61% accurate.

Diagnosing Alzheimer’s disease typically consists of undergoing several tests, including a patient’s medical history, physical examination, neurological examination, cognitive, functional and behavioral tests, and sometimes specialized cognitive tests. While the FDA approval of the blood test, which is called Lumipulse G pTau217/ß-Amyloid 1-42 Plasma Ratio, is a breakthrough, Dr. Honig notes that the test would not be for people who may be concerned about Alzheimer’s but not experiencing symptoms. Additionally, the blood test remains only one step in the diagnosis process.

“If the blood test came out positive, that’s a good sign to do confirmatory tests and try to see if it really is or isn’t Alzheimer’s disease,” says Dr. Honig. “It’s also exceedingly useful to have a blood test if we cannot do spinal fluid or PET scan on a patient.”

PET scans and spinal fluid tests remain the gold standards, but PET scans are costly and less accessible and spinal fluid tests require an experience practitioner to remove the fluid.

“The past two decades have brought impressive advances in the care of patients with dementia, with markedly improved diagnostic tools including PET scans, spinal fluid tests, and now new blood tests,” says Dr. Honig. “These diagnostic tests are particularly important, given the recent development of the first medications to slow down the progression of Alzheimer’s disease. We expect that additional research will bring greater ease in diagnosis and greater effectiveness in treatment therapies.”

Lawrence S. Honig, M.D., Ph.D., is a neurologist at NewYork-Presbyterian/Columbia University Irving Medical Center and professor of neurology at the Columbia University Vagelos College of Physicians and Surgeons, specifically in its Department of Neurology, the Taub Institute for Research on Alzheimer’s Disease and the Aging Brain, and the Gertrude H. Sergievsky Center, where he directs the New York State funded Center of Excellence for Alzheimer’s Disease. Dr. Honig’s clinical specializations focus on Alzheimer’s disease, Lewy body dementia, frontotemporal dementia, progressive supranuclear palsy, Creutzfeldt-Jakob disease, immune-mediated encephalitis, and other disorders of nervous system aging and degeneration.