Amazing as it might seem, doctors can detect and monitor diseases using molecules found in a sample of spit
Saliva carries different meanings around the world. In most of the United States, for example, the act of spitting is taken as an insult, whereas in some other cultures it can be considered a blessing. And although most Americans do not have a problem with the occasional wet kiss on the lips, they react with revulsion when they see objects covered with saliva. This seeming double standard arises because people perceive a difference between the saliva in their mouths and the saliva outside their bodies, according to Gordon Allport, a Harvard psychologist who penned an influential 1960 publication on the subject.
People won't drink their own saliva, explained Allport, who hypothesized that this fluid becomes nonself and alien to the mind the moment it exits the mouth. Maybe this aversion explains why the biomedical community has been slow to recognize that saliva doesn't just help one to chew and swallow—it also contains information about the physiological states of the body.
Like blood, saliva contains many protein and RNA molecules, both of which are encoded by genes. Scientists can identify several abnormal conditions if they know which genes are active and at what levels—information that can in many instances be gleaned from a sample of a person's blood. Saliva, however, is far easier and cheaper to collect and doesn't expose health-care workers to blood-borne diseases. Oral fluids are also simpler to handle because they don't clot, lessening the manipulations required. Furthermore, it's possible that diagnoses that use saliva could be made outside of a doctor's office, which is attractive for people who can't afford to see a physician or for people living in places where there are none.
Several tests that use saliva are already on the market. Three years ago, the U.S. Food and Drug Administration approved a product called OraQuick for detecting HIV-1 or HIV-2 infection. The assay, which is sensitive to anti-HIV antibodies in oral fluid or blood, indicates the result with one or two colored lines, similar to a home pregnancy test. The FDA authorized the use of OraQuick in clinical settings, but future versions may be sold over the counter. Commercially available kits can gauge the levels of a handful of hormones, including estrogen, testosterone and cortisol, from a sample of saliva. And other, yet unapproved, methods can screen for hepatitis viruses.
In addition to these simple measures, saliva has the potential to diagnose diseases with more complex origins, including cancer and diabetes. In recent years, my colleagues at the University of California, Los Angeles, and I have studied the RNA and protein molecules in saliva as indicators of disease. We've found that we can diagnose early-stage oral cancer and Sjögren's Syndrome, a systemic autoimmune disease marked by dryness of the mouth and eyes. Salivary diagnostics for serious illnesses affecting other parts of the body may be just around the corner. But to understand how biomedical researchers can perform such feats, one must first have a clear understanding of the makeup of this remarkable bodily fluid.