A blood test that measures levels of tau protein is nearly 98% accurate in ruling out Alzheimer's disease in people with cognitive impairment.Photo: Thomas Dierink, NCMIR/Science Photograph Library.
The US Food and Drug Administration (FDA) has approved a new blood test to help diagnose Alzheimer's disease. Test designed to rule out cognitive decline caused by Alzheimer's diseaseis the first blood test approved for use in primary care settings, but not the first to detect Alzheimer's disease.
Elecsys pTau181, developed by two pharmaceutical companies, Roche in Basel, Switzerland, and Eli Lilly in Indianapolis, Indiana, measures a specific phosphorylated form of tau, known as pTau181, in blood plasma. The test quantifies how much tau protein in the body has been altered as a result of Alzheimer's disease.
In an Oct. 13 press release, Roche said the Elecsys pTau181 test correctly ruled out Alzheimer's disease in 97.9% of cases in a clinical trial involving 312 participants. The test has a negative predictive value and helps rule out Alzheimer's disease in people with cognitive impairment, rather than providing a positive sign that someone has the disease.
Faster, cheaper, better: the rise in blood tests for Alzheimer's disease
“Numerous studies have shown that these blood measures have a high degree of agreement. They agree very well with the gold standard biomarkers for Alzheimer's disease that we have been using in specialist settings for many years,” says Ashwini Keshavan, a neurologist and biomarker researcher at University College London.
However, without full access to the test data, it is difficult to fully assess how accurate the test is, Keshavan said. “It would be important to see the sensitivity, specificity and positive predictive values to evaluate the accuracy of this test.”
How accurate is a blood test?
The pathology of Alzheimer's disease is characterized by the accumulation of sticky plaques of β-amyloid and tangles of tau proteins in brain tissue. Both proteins interfere with brain function, lead to the death of neurons and are responsible for the cognitive decline associated with the disease.1.
Another blood test for Alzheimer's disease has already been approved to measure these biomarkers in people's blood. Lumipulse, developed by Tokyo biotech firm Fujirebio, received FDA approval in May. This test measures the ratio of two proteins: pTau217 and amyloid-β(1–42). Alicia Algeciras-Schimnich, a clinical chemist at the Mayo Clinic in Rochester, Minnesota, says the two tests work slightly differently: The Lumipulse test can either give a positive result or rule out the presence of Alzheimer's disease.
In a study of 499 people, 97% of people who tested negative for the disease on the Lumipulse test had their results confirmed using follow-up diagnostic tests that measure levels of these proteins in cerebrospinal fluid and brain tissue. Nearly 92% of people who tested positive with Lumipulse for Alzheimer's disease had positive results on follow-up tests.
Color magnetic resonance imaging (MRI) of a healthy brain (left) and a brain with tau protein aggregates (right). Photo: Mark and Mary Stevens Institute for Neuroimaging and Computer Science/Science Photo Library
But no matter how accurate they are, blood tests prone to inconclusive results when linking cognitive symptoms to Alzheimer's disease. People may fall into a gray area and require further testing to make a clear diagnosis.
More than 30% of all samples analyzed using the pTau217 test developed by Quanterix, a life sciences company based in Billerica, Massachusetts, fell into this gray area.2. A similar number was not reported for the Elecsys or Lumipulse tests.
