The idea that a simple blood can diagnose Alzheimer’s disease is getting an enormous amount of attention. And someday, these tests may have important benefits. But before you run to your doctor demanding a blood draw, keep in mind that this test has important limitations.
Much of the buzz is over new research by Sebastian Palmqvist (paywall) of Lund University in Sweden and co-authors. It was based on a relatively small sample of about 1200 patients in that country. It looked at only one Alzheimer’s blood test, called PrecivityAD2, among many possible alternatives. It excluded patients already diagnosed with other forms of cognitive disease. And because it used a small sample in Sweden, it could not show how the tests fared with population subgroups, such as Black or Hispanic patients.
Within those limitations, the paper found this blood test was highly accurate. Yet, the test may be valuable to real-world patients in only a relatively narrow window of the progression of Alzheimer’s. And it may have only limited practical benefit since potential treatments for Alzheimer’s remain controversial at best. The paper was published in the Journal of the American Medical Association (JAMA).
A Breakthrough With Limited Practical Benefits
Like all tests for Alzheimer’s disease, these blood tests do not show the disease itself. They are not, for example, comparable to an x ray that shows a broken bone. Rather, they identify biomarkers that are linked to Alzheimer’s but are not the actual disease or even a proven cause of the condition. These biomarkers, proteins called amyloid beta (AB) and tau, often appear in the brains of people who never will develop symptoms of Alzheimer’s.
The purpose of the blood test is to determine whether diagnosed cognitive impairment is caused by Alzheimer’s or one of the many other conditions that result in memory loss and similar symptoms. The idea: Cognitive impairment plus the biomarkers of AB and tau probably means Alzheimer’s, rather than some other form of dementia.
Measuring Accuracy
Palmqvist wanted to measure the accuracy of this blood test against other ways to diagnose Alzheimer’s disease, such as PET scans, lumbar punctures (spinal taps) or cognitive testing by primary care doctors or specialists.
He found this biomarker blood test was about 90 percent accurate in diagnosing Alzheimer’s among patients in his study, a better record than the 61% accuracy for primary care and 73% accuracy in specialty care where doctors use standard cognitive tests and clinical exams. Lumbar punctures and especially AB PET scans have their own limitations in diagnosing Alzheimer’s. Palmqvist did not study other Alzheimer’s blood tests, including home tests.
Blood tests are less invasive than spinal taps and generally less costly than PET scans. However, the new study relied on a highly sophisticated tool for measuring results of the blood test—one that is not readily available to most clinicians and is more costly than more common methods.
What Will It Mean For Patients?
The authors acknowledge that more needs to be learned about how these biomarkers will influence patient care. But we do know that, given the current state of Alzheimer’s treatment, a simple blood test would benefit only a limited number of patients.
An early diagnosis of Alzheimer’s can be a valuable signal to for patients to change their lifestyle and begin to plan for a time when the disease becomes debilitating. On the other hand, it may increase anxiety without providing any clinical benefit.
The study also found that biomarker testing may be of limited value to those who feel they have experienced increased memory problems but have not received any clinical diagnosis. In general, this phenomenon, known as subjective cognitive decline, should not be ignored. But at that stage, biomarker blood tests may be no better at diagnosing Alzheimer’s than other methods.
In other words, if you occasionally forget where you left your car keys, don’t run out and get an Alzheimer’s blood test.
At the same time, these tests are not useful for people whose disease has progressed beyond the ability of current treatments to improve their quality of life.
Testing And New Drugs
That brings us to the closely related issue of those newly-approved monoclonal antibody drugs, such as Leqembi. These drugs clear those AB and tau proteins from the brain. But they do relatively little to slow the progression of Alzheimer’s disease.
These drugs are expensive and have dangerous side effects. And there is no evidence they provide any benefit for those with more advanced disease.
These all are reasons why a European drug regulator opposed sale of Leqembi for general use and why neurologists are so deeply divided over whether to prescribe them.
Like the new drugs themselves, these new biomarker blood tests are an important advancement for researchers, who are learning more about how Alzheimer’s works. But like those drugs, the benefits for real-world patients are much more modest than all the breathless hype suggests.
