saliva testing

Saliva testing or Salivaomics is a diagnostic technique that involves laboratory analysis of saliva to identify markers of endocrine, immunologic, inflammatory, infectious, and other types of conditions. Saliva is a useful biological fluid for assaying steroid hormones such as cortisol, genetic material like RNA, proteins such as enzymes and antibodies, and a variety of other substances, including natural metabolites, including saliva nitrite, a biomarker for nitric oxide status (see below for Cardiovascular Disease, Nitric Oxide: a salivary biomarker for cardio-protection).
Saliva testing is used to screen for or diagnose numerous conditions and disease states, including Cushing's disease, anovulation, HIV, cancer, parasites, hypogonadism, and allergies. Salivary testing has even been used by the U.S. government to assess circadian rhythm shifts in astronauts before flight and to evaluate hormonal profiles of soldiers undergoing military survival training.

Proponents of saliva testing cite its ease of collection, safety, non-invasiveness, affordability, accuracy, and capacity to circumvent venipuncture as the primary advantages when compared to blood testing and other types of diagnostic testing. Additionally, since multiple samples can be readily obtained, saliva testing is particularly useful for performing chronobiological assessments that span hours, days, or weeks. Collecting whole saliva by passive drool has a myriad of advantages. Passive drool collection facilitates large sample size collection. Consequently, this allows the sample to be tested for more than one biomarker. It also gives the researcher the ability to freeze the left over specimen to be used at a later time. Additionally, it lessens the possibility of contamination by eliminating extra collection devices and the need to induce saliva flow.

The clinical use of saliva testing occurred at least as early as 1836 in patients with bronchitis.
Testing the acidity of saliva occurred at least as early as 1808. The testing of salivation by the use of mercury was performed at least as early as 1685.

More recent studies have focused on detection of steroid hormones and antibodies in the saliva. Recent applications emphasize the development of increasingly sophisticated techniques to detect additional proteins, genetic material, and markers of nutritional status. According to Wong, scientists are now viewing saliva as "a valuable biofluid…with the potential to extract more data than is possible currently with other diagnostic methods."

Technique

Most saliva testing is performed using enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), high-resolution mass spectrometry (HRMS), or any number of newer technologies such as fiber-optic-based detection. All of these methods enable detection of specific molecules like cortisol, C-reactive protein (CRP), or secretory IgA. This type of testing typically involves collection of a small amount of saliva into a sterile tube followed by processing at a remote laboratory. Some methods of testing involve collecting saliva using an absorbent pad, applying a chemical solution, and monitoring for color change to indicate a positive or negative result. This method is commonly used as a point-of-care (POC) technique to screen for HIV. However, using absorbent pads and chemical solutions could very easily skew the results of immunoassays. Research by Dr. Douglas A. Granger and colleagues shows that outcomes for testosterone, DHEA, progesterone, and estradiol biomarkers are elevated when cotton-based collection materials are used as opposed to samples collected by other methods (i.e. passive drool). Researchers are currently examining the expanding role of saliva testing as part of routine dental or medical office examinations where saliva collection is simple to perform.

Physiologic basis

Humans have three major salivary glands: parotid, submandibular, and sublingual. These glands, along with additional minor salivary glands, secrete a rich mixture of biological chemicals, electrolytes, proteins, genetic material, polysaccharides, and other molecules. Most of these substances enter the salivary gland acinus and duct system from the surrounding capillaries via the intervening tissue fluid, although some substances are produced within the glands themselves. The level of each salivary component varies considerably depending on the health status of the individual and the presence of disease (oral or systemic). By measuring these components in the saliva, it is possible to screen for a variety of infections, allergies, hormonal disturbances, and neoplasms.

Clinical use

The following conditions are among those that can be detected through saliva testing (list not comprehensive): adrenal conditions (such as Cushing's disease/syndrome and Addison's disease), altered female hormone states (such as polycystic ovary syndrome COS menopause, anovulation, and hormonal alterations in cycling women), altered male hormone states (such as hypogonadism/ andropause and hyperestrogenic states), metabolic disturbances (such as insulin resistance, diabetes, and metabolic syndrome), benign and metastatic neoplasms (such as breast cancer, pancreatic cancer, and oral cancer), infectious conditions (such as HIV, viral hepatitis, amoebiasis, and helicobacter pylori infection), and allergic conditions (such as food allergy).

Uses in behavioral research

Saliva testing also has specific uses in clinical and experimental psychological settings. Due to its ability to provide insight into human behavior, emotions, and development, it has been used to investigate psychological phenomenon such as anxiety, depression, PTSD, and other behavioral disorders. Its primary purpose is to test cortisol and alpha amylase levels, which are indicative of stress levels. Salivary cortisol is a good stress index, with increased levels of cortisol correlating positively with increased levels of stress. Cortisol levels rise slowly over time and take a while to return to base level, indicating that cortisol is more associated with chronic stress levels. Alpha amylase, on the other hand, spikes quickly when confronted with a stressor and returns to baseline soon after the stress has passed, making salivary amylase measurement a powerful tool for psychological research studying acute stress responses. Samples are usually collected from participants by having them drool through a straw into a collection tube while experiencing a stimulus, with samples taken every few minutes to record the gradual change in stress hormone levels. Because the collection of saliva samples is non-invasive, it has the advantage of not introducing further stress on the participant that may otherwise distort results.