But how well do current blood tests work for that purpose?
People diagnosed with Alzheimer's disease face an incurable condition, but their outlook is marginally more hopeful than it was 10 years ago. That's because the first "disease-modifying" drugs for Alzheimer's, which target the disease's causes rather than its symptoms, have been approved in recent years.
Alzheimer's disease researchers hotly debate the exact causes of the condition, but they are united on the idea that early intervention is likely to produce the best possible outcome for patients. The aim is to remove disease-linked proteins before they can damage the brain.
The hope is that if these patients could be identified at this stage, they could access clinical trials and treatments when they might be the most helpful.
New blood tests for Alzheimer's can help patients get diagnosed earlier, which experts think could help boost the degree to which treatments help them. (Image credit: Iparraguirre Recio via Getty Images)What do Alzheimer's blood tests measure?
An additional test from Roche (the Elecsys pTau217 test) has received a basic safety certification in the European Union and U.K., which allows it to be sold but is not equivalent to FDA approval. This test is not approved in the U.S. yet. A second test from Fujirebio (the Lumipulse G pTau 217 Plasma assay) is in the same boat.
All of these tests in varying stages of approval measure "a core part of the pathology of Alzheimer's disease in the brain," said Dr. Sebastian Palmqvist, a clinical neurologist at Lund University in Sweden. These include two proteins that build up in the brains of Alzheimer's patients: tau and amyloid.
It's important to note that the blood tests alone aren't diagnostics for Alzheimer's disease, said Dr. Jemma Hazan, an old-age psychiatrist at University College London. Instead, "the test is telling you whether you have certain abnormal proteins in your brain." This, she said, makes it important to use the test in a setting where patients can be properly directed to the next steps of diagnosis and treatment.
"The clinicians working there [at memory clinics] have many, many years of experience of interpreting these types of biomarkers," he explained.
So currently, if a patient receives a positive blood test, they must undergo further cognitive testing, brain scans or invasive biomarker analyses to confirm the diagnosis.
The blood tests look for Alzheimer's-related proteins, including tau and amyloid, which form tangles in the brains of patients with the condition. (Image credit: wildpixel via Getty Images)
How well do the approved Alzheimer's tests work?
In clinical trials, the approved Lumipulse test had a sensitivity of 97.6% and a specificity of 90.8%. Sensitivity refers to the percentage of people who do have amyloid in their brains that test positive — so it's the rate of true positives. Specificity refers to the percentage of healthy people who test negative, meaning true negatives.
Analyses comparing pTau217 and pTau181, the two tau biomarkers that Roche uses in its tests, have concluded that pTau217 is a more accurate indicator of Alzheimer's disease. That means it more closely matches results from gold-standard amyloid brain scans.
Patients who take this more-accurate test are given one of three possible results based on two cutoff points, Harper said. If their tau level comes back above the upper cutoff — giving a positive result — clinicians can be "highly certain" that the patient has disease-linked amyloid in their brain, he said.
Someday, the blood tests could help flag patients in the earliest stages of Alzheimer's before they show any symptoms, scientists hope. (Image credit: stefanamer via Getty Images)
Current guidelines set by the Alzheimer’s Association require that patients end up in that gray area no more than 20% of the time, and the pTau217 test meets that requirement, Harper said. As with patients who get a positive test result, patients who receive these uncertain results are also advised to undergo further testing, such as scans or cognitive tests.
Harper said early data suggests that Roche's pTau217 test is roughly as accurate at detecting Alzheimer's disease proteins in cognitively unimpaired people as it is in those with symptoms. This may someday open the door to prevent Alzheimer's in patients who have not yet developed symptoms, he suggested, clarifying that Roche's test is not intended for that use yet.
How do Alzheimer's tests actually affect patients' care?
The approved antibody drugs are available through both private and government insurers, but they still cost the patient thousands of dollars each month. And again, these drugs only slow, rather than stop, the progression of the disease.
Diagnostic accuracy is important in its own right so that people feel more certain about the diagnosis and also there are management implications.
In the U.K., disease-modifying drugs like the antibodies have not been approved by the U.K.'s National Health Service (NHS) yet. Because the drugs only modestly slow Alzheimer's progression, ultimately, the NHS determined that they were too costly to cover without clearer signs of efficacy. That means British patients cannot access the drug for free.
"Diagnostic accuracy is important in its own right so that people feel more certain about the diagnosis and also there are management implications," Hazan said. While they can't access antibody drugs, patients do have access to drugs that help ease the condition's cognitive and behavioral symptoms, as well as non-pharmacological treatments like behavior management.
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Newer tests that are still in development may help reduce the likelihood of false positives by measuring only a subset of tau protein that comes directly from the brain rather than from the rest of the body, Palmqvist said. But for now, mass screening remains a goal, rather than a reality, for these blood-based Alzheimer's diagnostics.
This article is for informational purposes only and is not meant to offer medical advice.
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