Progress Toward a Saliva-based Test for Alzheimer Disease
Two recent pilot studies suggest that it may be possible to develop a saliva-based test for Alzheimer disease, perhaps even before symptoms appear. Although there is currently no cure for Alzheimer disease, identifying affected individuals earlier may allow for better responses to various treatments and support services when symptoms are still mild. Although they are both proof-of-concept studies, their results are encouraging because they indicate that it may eventually be possible to diagnose Alzheimer disease by a simple, saliva-based test.
Currently, there is no single diagnostic test for Alzheimer disease. The diagnosis is primarily based on clinical symptoms, such as changes in memory and cognition. It is possible to look for changes in the levels of two proteins that accumulate in the brain in Alzheimer disease (amyloid β42 and Tau) in the cerebrospinal fluid, but this testing is invasive and still not definitive. Even for individuals who face an increased risk of Alzheimer disease due to genetic risk factors, such as APOE e4 genotype or mutations in the PSEN1 gene, genetic testing does not help diagnose the presence of disease. That is why researchers are working toward developing non-invasive, accurate, and affordable screening tests for Alzheimer disease.
In the most recent study in the Journal of Alzheimer’s Disease, researchers from a Canadian company described a method for measuring a fragment of the amyloid β42 (Aβ42) protein in saliva. Individuals with Alzheimer disease slowly develop harmful deposits of the full-length form of this protein (called amyloid-β precursor protein) in the brain. The researchers measured Aβ42 in the saliva 37 individuals: 7 with Alzheimer disease and 30 healthy individuals.
The researchers found that levels of Aβ42 were nearly twice as high in the people with Alzheimer disease versus the healthy individuals. They also noted that three individuals with an increased risk for developing Alzheimer disease due to genetic causes or a strongly positive family history also had higher levels of Aβ42, suggesting that Aβ42 levels may increase before symptoms of Alzheimer disease are detectable.
The second study, also published in the Journal of Alzheimer’s Disease, was performed by a group of researchers from the Beaumont Research Institute and the University of Alberta. Hoping to identify metabolite changes specific for Alzheimer disease, they measured naturally-occurring chemical substances (metabolites) in saliva from three groups: 9 people with Alzheimer disease, 8 people with mild cognitive impairment (i.e., mild changes in memory and thinking abilities that do not compromise day to day activities), and 12 healthy people.
The researchers found metabolite patterns that allowed them to differentiate each of the three subject groups. According to the researchers, this pilot study shows the potential for using the measurement of metabolites in saliva for earlier diagnosis of Alzheimer disease.
It's important to remember that results from these studies are based on very small numbers of individuals and that they will need to be replicated in much larger groups comprised of people with a variety of similar conditions. While encouraging, test results in small-scale, proof-of-concept studies are often disappointing when they are tried in larger groups of people under real-world conditions.