The results of this study demonstrate and reinforce the feasibility of prospectively collecting and analyzing saliva for the detection and quantification of salivary Aβ42. Our analysis of the quantification of Aβ42 in the saliva of patients with mild to moderate AD compared to Aβ42 in the saliva of controls demonstrated that AD patients had a 2.45-fold increase in Aβ42. Thus, the saliva levels of Aβ42 were significantly higher (t20 = 12.8, p = 2.8 × 10− 4) in AD patients than in controls. This adds to the previous reports  in several ways. First, it demonstrates the feasibility of salivary specimen collection for the detection of Aβ42. Second, it shows that the assay is reproducible in that the cohort used in this report does not overlap at all with the first report. Third, the current cohort uses a prospectively characterized clinical sample.
The use of salivary Aβ42 as a biomarker for AD is highly innovative compared to more traditional detection methods (cerebrospinal fluid, imaging, and blood studies). The results of this simple, non-invasive test indicate its potential use as an adjunct for the diagnosis of AD or as a screening measure for its negative predictive value to exclude patients with a low probability of disease progression.
The test described measures salivary levels of Aβ terminating at position 42 (Aβ42). Previous studies have found Aβ42 to be produced in other organs [Lee et al. 2017 7], thus establishing the generality of its production. Brain deposits of this peptide are characteristic of AD. Biomarker studies indicate that these brain deposits commence a decade or more before the clinical onset of AD. Similar to the results of other studies, [7, 8] our findings indicate that Aβ42 is elevated in the saliva of patients with AD. Thus, our data support elevated salivary Aβ42 as a potential biomarker for AD. The current study replicates the findings of Lee et al.  is the amount of amyloid detected is > 2× higher in AD compared to NC. These differences are larger than the differences reported by Bermejo-Pareja  possibly because the assay method differs somewhat.
Our report contrasts with previous studies. Shi et al.  used high sensitivity mass spectrometry to detect salivary Aβ and tau. They found that salivary tau was elevated but Aβ42 was not detectable. How can these findings be explained? Mass spectrometry is able to detect the molecular weights of the proteins. Since there are multiple forms of Aβ, the mass spec might not be detecting the species of Aβ that we are detecting in our study. The main difference in accounting for the detectable Aβ in our study is that we used an ELISA assay with antibodies specifically able to bind to Aβ42.
What these results do not indicate is the source of the Aβ. Is Aβ42 endogenous to the salivary gland? Future studies might want to explore the histology of salivary glands to determine if salivary glands produce Ab forms similar to the CNS. Amyloid is produced in detectable amounts in a variety of organs . Identifying the source will be important in determining the clinical utility.
In this study, we find that age did not affect the findings even when controlled for. The group of controls in our study was significantly younger than the group of AD patients (60.4 ± 4.7 years vs. 77.8 ± 1.8 years, p < 0.05). In contrast, in the Lee et al.  study, almost identical levels of salivary Aβ42 were found in AD patients and controls aged 15 to 92 years.
Does salivary Aβ42 reflect the level of Aβ in the central nervous system? Does salivary Aβ42 come from another source? Future studies will explore the source. Future studies will involve collecting saliva specimens across all adult age groups to analyze, compare, and determine the stability of Aβ42 by age. Future studies will also assess the specificity and sensitivity of the salivary test in patients with mild cognitive impairment, Lewy body disease, primary progressive aphasia, and Parkinson disease. Other studies to be completed include test-retest validity, and multi-lab assay validity for standardization.
Ultimately, the results of the salivary test for an individual will be correlated with the amyloid status of that person. Future studies will address a variety of questions: Is amyloid positivity associated with higher levels of Aβ42? Can the assay method be adapted to detect and quantify other biomarkers, such as tumor necrosis factor-α, interleukin-6, and interleukin-1β? Do salivary levels of Aβ42 change with or correlate with the severity of dementia? Even more development is required, including test-retest validity, multi-laboratory validation, and identification of confounders of diurnal variations. Given the strength of the results from in this study, salivary Aβ42 warrants further investigation as a potential biomarker for mild to moderate AD.