Using Saliva as a Diagnostic Fluid: 5 Triumphs for Salivary Biomarkers
For most people, saliva is written off as an inert (kinda gross) fluid, necessary for food digestion. But for researchers, it’s increasingly being appreciated for its molecular complexity and the potential for DNA & RNA extraction and analysis, proteins isolation, and small molecules to be used as biomarkers for oral and systemic diseases. While the diagnostics community has long been hesitant, over the past two decades, acceptance of saliva as a diagnostic fluid has grown.
The COVID-19 pandemic response has helped further the cause, raising additional awareness about the diagnostic uses of saliva, serving as a proof point for diagnostics developers considering saliva as a sample type. While blood, urine, and nasopharyngeal swabbing remain favorites in the clinical setting, there are many other diagnostic areas where saliva testing has been heavily researched and incredibly helpful for detecting oral and systemic health issues.
Let’s look at some of the application areas where researchers are making the most headway into the successful use of saliva as a diagnostic fluid.
For systemic infectious diseases, sensitive and selective detection, early in infection, can be critical to preventing morbidity and mortality. The use of salivary diagnostics for the detection of infectious diseases, along with the many advantages of saliva sample types has made it a major focus for the development of diagnostics.
One of the early uses of saliva as a diagnostic fluid was for HIV detection. A number of different assays, with high sensitivity and selectivity, were developed to detect IgG or IgA antibodies against specific HIV protein epitopes.1-3 An FDA-approved, commercially available HIV test, OraSure®, detects salivary antibodies against the p24 antigen of HIV.4 Rapid tests for the detection of hepatitis C virus,and other viral saliva biomarkers, including anti-virus antibodies (i.e., IgG, IgA, and IgM) or RNA, have been developed for hepatitis A, hepatitis B, dengue, Ebola, Zika, measles, mumps, and rubella.6 Recently, SARS-CoV-2 RNA tests using saliva have joined the list of successes for infectious disease detection and several studies have indicated a high concordance with the “go-to” sample type, nasopharyngeal swabs.7-9
As “omics” techniques have gained traction in nearly every facet of biomedical (and other) sciences, saliva, as a sample type has not been spared. Accordingly, the Salivaomics Knowledge Base was established to act as a data repository for human salivary “omics” data.10 A growing appreciation for the microbiome in human health, has enticed researchers to focus on the oral microbiome, using saliva as a sample type to provide a snapshot of the microbial species present, in search of microbial biomarkers for oral and systemic diseases.
Despite some analytical challenges, since 90% of the DNA in a saliva sample is host (human) DNA,there have been a number of promising discoveries that may hold promise for the future. For instance, children living with autism spectrum disorders were found to have a significantly higher abundance of bacterial pathogens (such as Haemophilus) and reduced abundance of commensals (including Prevotella, Fusobacterium, and others), compared to children without autism.12 As with other “omics” techniques, these results have potential but oral microbiome analysis doesn’t yet have the precision, accuracy, or sensitivity required for the clinical spotlight.13
Cancer is a major cause of morbidity and mortality and is on the rise worldwide.14 Developing diagnostics that can detect cancer early, predict clinical progression, or track therapeutic response is a major focus for researchers and a strategy that can limit the impacts of the disease. Molecular diagnostics for cancer biomarkers have long been an integral part of disease detection, treatment selection, disease monitoring, and tracking metastasis.15
Because saliva holds many advantages over liquid or tissue biopsies, which are commonly used in cancer diagnostics, there has been significant effort to apply genomics, proteomics, and metabolomics to detect DNA, RNA, protein, and other biomarkers.16 Thus far, salivary biomarker studies have focused mainly on brain, esophageal, gastric, head and neck, lung, ovarian, pancreas, prostate, and breast cancers.16 While there have been many promising findings, the possibility of salivary biomarkers as an alternative to liquid biopsy for early cancer diagnosis and treatment monitoring is still evolving. Promising salivary biomarkers have been identified, yet the mechanism by which they enter the saliva and their role in oncogenesis are still in the discovery phase.16
Drug Abuse and Therapeutic Monitoring
Unbound or free drugs – both illicit and therapeutic – are excreted into oral fluids and concentrations in saliva correlate closely with free drugs in plasma. For this reason, saliva collection and several assessment methods have been used for the detection of drugs of abuse as well as therapeutic monitoring for psychotropic medication.17,18
Testing for Drugs of Abuse
The simplicity and rapid nature of saliva testing make it particularly applicable to workplace and roadside drug testing. Several commercially available collection devices are available for cannabinoids, opioids, cocaine, amphetamine, and benzodiazepines along with LC- or GC-MS-based methods of detection. While salivary testing for drugs of abuse has provided reliable alternatives to blood and urine testing, there is still a need for additional research into confounding factors, such as time since sample collection and since last drug use, and their effect on diagnostic test results.17
Saliva testing has also been particularly useful for treatment monitoring for psychiatric medication. For patients living with bipolar disorder and other psychiatric diseases, noncompliance with prescribed treatment regimens is common and correlated with poor outcomes, such as increased ER visits and worse functioning.18 In addition, therapeutic windows may vary from patient to patient or be very narrow, as is the case with lithium and valproic acid.
Monitoring drug concentrations through saliva is particularly helpful and has become more accurate, reliable, and highly sensitive.18 Studies on salivary detection of lithium, valproic acid, carbamazepine, lamotrigine, antidepressants, and antipsychotics have been done with varying degrees of success and correlation with serum levels have demonstrated significant promise. However, there are still several challenges and considerations, such as a lack of standardization and a large variation in other confounding variables (i.e., polypharmacy, collection and processing protocols, dehydration, etc.).18
It is well accepted that hormones, such as cortisol or testosterone, move from plasma into the saliva or can be produced directly by salivary glands, as is the case with salivary melatonin or cortisone, a precursor to cortisol.19
Cortisol is the most studied and used test in the salivary biomarker field and several studies correlate salivary cortisol with pain or stress in both humans and animals.19 Testosterone is another hormone that has been well studied and found to correlate with the presence of depression, anxiety disorders, and aggressive behavior.19 On the other side of the emotional spectrum, salivary oxytocin has been found to be a reliable biomarker for human wellbeing.19
More Triumphs for Saliva as a Diagnostic Fluid
Over the next decade, researchers will continue to discover more accurate and reliable salivary biomarkers and develop diagnostic tests to sensitively and selectively detect them. For more rapid innovation, products that streamline workflows and save diagnostics labs time and money are needed.
Thermo Fisher Scientific developed the SpeciMAX Dx™ Collection Kits for simplified self-collection of saliva samples for SARS-CoV-2 testing and many other application areas. The collection tubes can be paired with an automated liquid handler to eliminate messy manual transfer steps and use incubator, storage, or refrigeration space efficiently.
Read more related articles:
- How to Get More Out of Your Saliva Collection & Handling
- The Future of Saliva Biomarkers in the Clinic
Want to learn more about Saliva as a Sample type? Start from the beginning.
- What is saliva and why is it important for diagnostics?
- 5 Benefits of Saliva Testing: Why it’s the Next Big Thing for In Vitro Diagnostics
This article contains product information intended for General Laboratory Use. It is the customer’s responsibility to ensure that the performance of the product is suitable for customer’s specific use or application.
- Scully C. HIV topic update: salivary testing for antibodies. Oral Dis. 1997;3(4):212-215.
- Scully C, Samaranayake LP. Clinical virology in oral medicine and dentistry. Cambridge University Press, Cambridge;1992.
- Matsuda S, Oka S, Honda M, Takebe Y, Takemori T. Characteristics of IgA antibodies against HIV-1 in sera and saliva from HIV-seropositive individuals in different clinical stages. Scand J Immunol. 1993;38(5):428-434.
- Malamud D. Oral diagnostic testing for detecting human immunodeficiency virus-1 antibodies: a technology whose time has come. Am J Med. 1997;102(4A):9-14.
- Cha YJ, Park Q, Kang ES, et al. Performance evaluation of the OraQuick hepatitis C virus rapid antibody test. Ann Lab Med. 2013;33(3):184-189.
- Martínez-Subiela S, Cantos-Barreda A. Chapter 11: Salivary Diagnosis of Infectious Diseases. In: Tvarijonaviciute A, Martínez-Subiela S, López-Jornet P, Lamy E, eds. Saliva in Health and Disease. 1st ed. Springer Nature Switzerland; 2020:221-246.
- Bastos ML, Perlman-Arrow S, Menzies D, Campbell JR. The Sensitivity and Costs of Testing for SARS-CoV-2 Infection With Saliva Versus Nasopharyngeal Swabs: A Systematic Review and Meta-analysis. Ann Intern Med. 2021;174(4):501-510.
- Butler-Laporte G, Lawandi A, Schiller I, et al. Comparison of Saliva and Nasopharyngeal Swab Nucleic Acid Amplification Testing for Detection of SARS-CoV-2: A Systematic Review and Meta-analysis [published correction appears in JAMA Intern Med. 2021 Mar 1;181(3):409. JAMA Intern Med. 2021;181(3):353-360.
- Vogels CBF, Brackney DE, Wang J, et al. SalivaDirect: Simple and sensitive molecular diagnostic test for SARS-CoV-2 surveillance. Preprint. medRxiv. 2020;2020.08.03.20167791.
- Shah S. Salivaomics: The current scenario. J Oral Maxillofac Pathol. 2018;22(3):375-381.
- Marotz CA, Sanders JG, Zuniga C, Zaramela LS, Knight R, Zengler K. Improving saliva shotgun metagenomics by chemical host DNA depletion. Microbiome. 2018;6(1):42.
- Qiao Y, Wu M, Feng Y, Zhou Z, Chen L, Chen F. Alterations of oral microbiota distinguish children with autism spectrum disorders from healthy controls. Sci Rep. 2018;8(1):1597.
- Franco-Martínez L, Rubio CP, Contreras-Aguilar MD. Chapter 4: Methodology Assays for the Salivary Biomarkers’ Identification and Measurement. In: Tvarijonaviciute A, Martínez-Subiela S, López-Jornet P, Lamy E, eds. Saliva in Health and Disease. 1st ed. Springer Nature Switzerland; 2020:221-246.
- Sung H, Ferlay J, Siegel RL, et al. Global Cancer Statistics 2020: GLOBOCAN Estimates of Incidence and Mortality Worldwide for 36 Cancers in 185 Countries. CA Cancer J Clin. 2021;71(3):209-249.
- Sokolenko AP, Imyanitov EN. Molecular Diagnostics in Clinical Oncology. Front Mol Biosci. 2018;5:76.
- Navazesh M, Dincer S. Chapter 19: Salivary Bioscience and Cancer. In: Grander DA, Taylor MK, eds. Salivary Bioscience. 1st ed. Springer Nature Switzerland; 2020:449-467.
- Navazesh M, Ahmadieh A. Chapter 16: Saliva and Drugs of Abuse. In: Grander DA, Taylor MK, eds. Salivary Bioscience. 1st ed. Springer Nature Switzerland; 2020:371-393.
- Thomas EA. Chapter 17: Therapeutic Drug Monitoring in Saliva. In: Grander DA, Taylor MK, eds. Salivary Bioscience. 1st ed. Springer Nature Switzerland; 2020:395-417.
- Escribano D, Tecles F. Chapter 14: Salivary Biomarkers in Welfare Studies. In: Tvarijonaviciute A, Martínez-Subiela S, López-Jornet P, Lamy E, eds. Saliva in Health and Disease. 1st ed. Springer Nature Switzerland; 2020:293-319.
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