A blood test to diagnose Alzheimer’s disease moved closer to reality this week after new findings were announced at the Alzheimer’s Association International Conference on July 29, 2020. The test showed extremely high accuracy – around 90% – for detecting chemicals in the blood that are specific for Alzheimer’s.
Those who treat patients with Alzheimer’s say that the tests need only a bit higher level of accuracy before they can be used clinically, which could be in two to three years. This breakthrough could perhaps allow doctors to not only identify symptomatic patients with the disease, but also to identify people with no symptoms who are at risk of developing the disease, and thus begin interventions.
About 5.7 million people in the U.S. live with Alzheimer’s, but that number could triple by 2050, the Alzheimer’s Association estimates.
While blood tests have been slowly increasing their diagnostic accuracy, the new blood test – analysing the amount of a brain protein, p-217, in the blood – appears to be accurate in over 90% of cases in a study looking at blood samples from people with definite Alzheimer’s disease. Accuracy rates of other tests will likely increase
over time. But this result shows that a breakthrough test is indeed possible. Before
the tests are available to the public through FDA approval, we’ll need another two to three years to complete the studies.
As researchers who have spent our professional lives studying this disease and treating patients with it, we think this news is especially important. It represents a significant leap forward in our ability to use peripheral blood tests for detection of Alzheimer’s and possibly as a marker of effectiveness in developing medical treatments. Here is why.
Have we grabbed the ‘brass ring’ for diagnosis?
Just one year ago, we wrote a piece for The Conversation on blood tests for Alzheimer’s disease, ending it with the hope that several promising blood tests would soon emerge as accurate and specific. Now, it appears they have. The tests have been centred on the ability to test for either beta amyloid or tau, the characteristic proteins that are deposited in the brain in Alzheimer’s disease, and the tau tests lagged behind the beta amyloid tests. Now tau testing has jumped into the lead.
Until the early 1990s, with the routine use of brain MRI scans, it was difficult to be certain whether a person with cognitive loss had Alzheimer’s. Even the best neurologists would get the diagnosis wrong about one in four times. MRIs increased accuracy; it could show vascular disease and atrophy characteristic of Alzheimer’s or other dementias, but could not confirm the diagnosis with certainty. Diagnosis was even harder in people over 80, where the changes in thinking and memory with ageing were not always easy to separate from early Alzheimer’s symptoms, and normal age-related atrophy made differentiation from disease-based brain shrinkage more difficult.
Until this century, the only definitive diagnosis of the disease occurred after death, at autopsy, by finding certain levels of two specific lesions, or areas of abnormal tissue. Those two lesions are beta- amyloid plaques and neurofibrillary tangles.
And it was not unusual to find, following autopsy, that someone diagnosed clinically with Alzheimer’s disease had another neurodegenerative disease, disease related to blood vessels in the brain, or some combination of these.
Marching toward answers
Over the last two decades, however, the medical field has made progress in detecting the disease by identifying specific diagnostic biomarkers, or biological signs of disease. MRI scans helped by showing shrinkage of the areas of the brain that underlie memory. But they are not specific for Alzheimer’s.
Two key biomarkers, amyloid protein, found in plaques, and tau protein, found in tangles, became the targets outside of the brain tissue itself, since their presence in the brain defines the disease.
With the identification of these biomarkers, doctors could test patients to see if either amyloid or tau, or both, were abnormal in patients in whom they suspected Alzheimer’s. But the testing has not been easy or cheap.
One way was a spinal tap, whereby doctors could obtain cerebro-spinal fluid, the fluid around your brain and spine, and measure levels of tau and amyloid, which change if the disease is present. While doctors consider this procedure safe and routine, it is not a favourite among patients.