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Saliva is a fluid found in the mouth, it is made up of a mixture of products secreted from the salivary glands1. Proteins and various molecules from different sources make up the whole saliva. The many functions of the saliva like its aid in speech, help in maintenance of oral and general health, and help in digestion of food are due to its physicochemical and biochemical attributes. In the last few years, there has been an increase in the use of saliva as a diagnostic tool because it has been discovered that some viral, bacterial and systemic diseases can be diagnosed with the aid of saliva testing2. This feature of the saliva as a clinical tool can be due to its origin, its composition, its functions and its ability to interact with other organs in the system3.
HOW IS SALIVA COLLECTED?
When in comparison with blood collection, saliva collection is a simple and non-invasive method, it is easy to store, and it is not expensive to collect. With the addition of modern techniques and chemical instrumentation equipment, there has recently been an increase in its use for laboratory research, applicable for the basic and clinical purposes in dentistry and other medical areas4-7.
SALIVA USE IN MONITORING HUMAN HEALTH
In recent times, saliva has also been seen as a mirror of the body, and therefore its use for diagnostic purposes has been in many ways, some of which are:
Screening for Viral and bacterial infection: Saliva contains a little amount of Immunoglobin G (IgG), which is mainly gotten from crevicular fluid and the mucosal transudate. Saliva with a higher IgG content can be used in the screening of viral infections in the human body. Recently, a correlation has been found between the salivary and serum antibodies, used in the detection of Human immunodeficiency Virus (HIV) 8, and this was because research had shown that if blood has been contaminated with the HIV virus, there is a higher chance of the virus been detected in the saliva 9. Salivary antibodies have also been found after immunization against viruses like poliovirus, rotavirus and Hepatitis A virus10. Because of this, saliva testing has been found to be one of the good ways for evaluation of the response of the body immunity to vaccinations against these viruses9.
Cancer: A group of American scientists found out that markers for breast cancer are present in the human saliva, and the mix of protein secreted by the salivary gland is changed based on the appearance of breast cancer, in that the salivary gland of a person with breast cancer secrets a different protein profile to a person without breast cancer21. In a breast cancer research, the salivary level of the soluble fragments of c-erbB-2 protein and 15-3 cancer antigen were found to be high in cancer patients than in healthy people11. In profiling of salivary RNA transcript with microarray analysis, several genes have significantly different expression levels in the comparison of patients with oral squamous cell carcinoma (e.g. pharyngeal cancer) and control patients.12.
Tests for drug/substance abuse: In previous years, test for substance abuse has always been done with the blood plasma, but recently, a group of scientists developed a method used to measure the level of the epilepsy drug (levetiracetam) in the saliva13. Other substances that can be tested for in the saliva include tobacco, alcohol, barbiturates, opoids, lysergic acids, cocaine, amongst many others14. Saliva testing for substance abuse is very straight forward, because it gives either a positive or negative outcome. A Research to see the possibility of saliva detecting protein compounds has been proposed and this will be of an important use in sports for the anti-doping tests4.
DNA testing: Ideally, the source for genomic DNA is in the blood, but recently, DNA testing in saliva has increasingly been investigated, and it saliva has been found to be a good DNA source from the oral cells. This DNA testing can be used for paternity proofs, forensic testing and profiling of biomarkers. 11, 15.
Control of Cardiovascular diseases: One of the highest death causing diseases are cardiovascular disease16, and the marker amylase is one of those found in saliva. Amylase has always been used as a control for patients who have had cardiovascular surgeries3. A research carried out by Adam et al showed that the low levels of the salivary amylase found in patients with aorta aneurism can be linked to be a cause of death increase in these patients16.
Sialochemistry analysis: Gonzaz et al. found out that saliva can be a good medium for early monitoring of an exposure to lead and cadmium, since salivary levels of these elements arise from the diffusible fraction of plasma17.
There is therefore an increasing interest on the diagnostic role of saliva but there are only very few studies that utilize saliva testing in the diagnosis of diseases such as alcohol cirrhosis and cystic fibrosis18, 19.
Saliva testing has been found to be a very useful diagnostic tool; it is a biological fluid that offers qualitative answers in diagnosis, toxicology and in forensic science. Nevertheless, salivary assays are still not frequently used compared to blood plasma and serum assay, as it is an upcoming research area for some major health related infections. Oral fluid being the 'mirror of body' is a perfect medium to be explored for health and disease surveillance20, and another reason why this area of research must be developed is apart from saliva been found to contain markers of some diseases, it is easier to collect and easier to preserve, therefore, it should be encouraged in major medical laboratories for diagnosis.