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.
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 testing of salivation by the use of mercury was performed at least as early as 1685. Testing the acidity of saliva occurred at least as early as 1808. The clinical use of saliva testing occurred at least as early as 1836 in patients with bronchitis. In 1959, scientists in the journal Cancer raised the possibility of using biochemical changes in acid phosphatases in saliva as an indicator of the presence of prostate cancer.
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, polymerase chain reaction, high-resolution mass spectrometry, 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, 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 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. 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. 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 : adrenal conditions, altered female hormone states, altered male hormone states, metabolic disturbances, benign and metastatic neoplasms, infectious conditions, and allergic conditions.

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.
In more specific studies looking at the link between cortisol levels and psychological phenomena, it has been found that chronic stressors such as life-threatening situations, depression, and social or economic hardship correlate with significantly higher cortisol levels. In situations where a subject undergoes induced anxiety, high cortisol levels correspond with experiencing more physiological symptoms of nervousness, such as increased heart rate, sweating, and skin conductance. Additionally, a negative correlation was discovered between baseline levels of cortisol and aggression. Salivary cortisol levels can thus provide insight into a number of other psychological processes.
Alpha amylase levels in saliva provide a non-invasive way to examine sympathoadrenal medullary activity, which can otherwise be measured with electrophysiological equipment or blood plasma readings. Salivary alpha amylase levels have been found to correlate with heightened autonomic nervous system activity levels, reacting in similar ways to the hormone norepinephrine. Subsequent findings reveal a relationship between α-amylase and competition. Results showed that alpha amylase levels changed when reacting to competition, but not when anticipating it. Furthermore, by testing alpha amylase levels, scientists noticed a difference in reactivity behavior among individuals with previous experience in a similar situation.
While saliva testing has the promise of becoming a valuable and more widely used tool in psychological research in the future, there are also some disadvantages to the method that must be kept in mind, including the cost of collecting and processing the samples and the reliability of the measure itself. There is a substantial amount of both within-person and between-person variability in cortisol levels that must be taken into account when drawing conclusions from studies.
Many studies have been performed to further examine the variables that contribute to these within-person and between-person variances. Analyses of the variables that affect cortisol levels has yielded an extensive list of confounding variables.
Diurnal variation is a major factor for within-person variance because baseline cortisol levels have been known to differ based on the time of day. For normally developing individuals who follow a typical day–night schedule, cortisol production peaks during the last few hours of sleep. This peak is thought to aid in preparing the body for action and stimulate the appetite upon waking. Diurnal variation is also affected by psychological conditions. For example, Early morning cortisol levels have been found to be elevated in shy children and late night levels elevated in depressed adolescents, particularly the between the hours of two and four PM. This might be important for understanding emotions and depressive symptoms.
Other variables that affect within- and between-person variation are listed below. The list is not meant to be comprehensive and the impact of many of these variables could benefit from further research and discussion.

Age is one of the major factors for between-person variance. Some studies indicate children and adolescents exhibit greater cortisol activity potentially related to development.
Gender has been found to impact base line levels of cortisol, contributing to between-person variance. In generally stressful situations, levels of cortisol in males have been found to increase to nearly double the amount when compared to females. In stressful social situations, however, women but not men tend to show significantly higher levels of cortisol.
The menstrual cycle has been found to impact levels of cortisol in the body, impacting both within- and between-person variance. Women in the luteal phase reportedly have levels of cortisol equal to men, suggesting no sex differences in base levels of cortical when women are not ovulating. Women in the follicular phase and women taking oral contraceptives reportedly have significantly lower levels of cortisol when compared to men and women in the luteal phase.
Pregnancy has been found to increase levels of cortisol in the body. In particular, breast-feeding has been found to decrease levels of cortisol in the short-term even if a mother is exposed to a psychosocial stressor.
Nicotine is known to increase levels of cortisol in the body since it stimulates the HPA axis. After at least two cigarettes, smokers show significant elevations of salivary cortisol levels. Furthermore, habitual smokers show blunted salivary cortisol responses to psychological stressors.
Food has been found to affect levels of cortisol. The presence of proteins has been found to increase cortisol. This variable is often affected by diurnal variation, with cortisol being notably higher during lunchtime than dinnertime, and gender, with women having higher levels of cortisol after eating than men.
While some studies examining the effects of alcohol consumption and caffeine intake on base levels of cortisol have found positive correlations, the results are mixed and would benefit from further examination.
Intense or prolonged exercise can result in increased levels of cortisol. Short-term and low-level exercising only mildly increases levels of cortisol.
Repeated exposure to initially stressful stimuli has been found to result in a leveling off of cortisol in the body.
Birth weight has been shown to be inversely related to base levels of cortisol; low birth weight is correlated with high levels of cortisol.
Position within a social hierarchy has been found to affect levels of cortisol. One study in particular looked at a sample of 63 army recruits and found that socially dominant subjects showed high salivary cortisol increases compared to only modest elevations in subordinate men after stress exposure and physical exercise.
Some medications have been found to affect levels of cortisol in the body but the results from studies examining these affects have been mixed. The impact of medications on cortical levels could benefit from further research.