Since the discovery of PCR and other molecular techniques, the use of DNA as a building block for diagnostics has grown rapidly. Market sources estimate that more than 500 million molecular tests are done annually in the United States (2010 numbers) and that this number will grow to 750 million by 2015 [28, 57].

Other statistical reports estimate that the worldwide market for molecular diagnostics was $5.5 billion in 2013 and on a growth curve [58]. Already a small fraction of the estimated 500–750 million tests use saliva as a sample source, particularly in the research and life sciences environments, but trends indicate that as current studies are published confirming the efficacy of saliva as an ideal specimen, the proportion of oral-based tests will rise sharply. In addition, new high-profile research projects targeting salivary RNA (including mRNA and miRNA) and proteins (proteomics) will magnify the interest in oral testing, resulting in new diagnostic areas where saliva will be a specimen of choice. A significant example of this is the recent grant award of more than $5 million to the Wong group at UCLA to examine extracellular RNA in exosomes and other microvesicles in gastric cancer [29].

A recent publication by Gallo et al. [59] has surprisingly shown that the majority of microRNAs detectable in serum and saliva are concentrated in exosomes, so this is likely to lead to a focus in this area of salivary research.

The literature supports the widespread use of saliva as an ideal medium for SNPs, genotyping, microarrays, genome-wide association studies (GWAS), and other molecular technologies; however, most of the current applications are confined to the life sciences research area. At this time the only clinical test using saliva specimens on an automated platform is the eSensor Warfarin Sensitivity Test, which uses samples collected using the OraGene Saliva DNA Device. Saliva samples are pipetted into a cartridge that fits into the eSensor XT8 multiplex PCR system. Microfluidic chambers in the cartridge deliver diluted specimens to the PCR reaction site. The eSensor XT8 delivers results for multiple genetic mutations (in this case VCORC-1 and CYP2C19) in 30 min.

A number of other tests have been validated to saliva and are available as Lab Developed Tests (LDTs) in the United States. These tests are run in CLIA (Clinical Lab Implementation Amendments Act 1988) certified laboratories, who perform internal validations and obtain state approvals to begin running the tests. This list is not exhaustive, but some of the tests/platforms validated to saliva specimens include human papillomavirus (HPV) and periodontal disease detection at Oral DNA Laboratories (Brentwood TN, now part of Access Genetics), personal genomic profiling at 23andMe (Mountain View, CA), and whole genome sequencing at the Personalized Genome Project (PGP) headed by Dr. George Church at Harvard University. The Mayo Clinic now runs a series of genotyping tests including CYP2C19, CYP2D6, HLA B1502, HLA B5701, UGT1A1, and many others using a single saliva sample. Samples are analyzed using sequencing and single gene/gene mutation techniques.

The literature abounds with new applications for saliva, too many to provide an exhaustive list, but some of the newer molecular tests to be validated to saliva include the InPlex Cystic Fibrosis Test from Hologic (Bedford MA), which simultaneously detects 23 mutations in the cystic fibrosis transmembrane receptor (CFTR) gene and the IVS8/5T/7T/9T markers, the Asuragen AmplideX FMR1 Gene test for Fragile X Syndrome in autism and the multiparameter Affymetrix GeneChip Scanner 3,000 Targeted Genotyping System, capable of detecting close to 3,000 SNPs. In this latter example, saliva was shown to be equivalent or better than blood for genotyping 2,918 SNPs from Human Ch12 (developed during the HapMap Project). Positive results for saliva were also observed with the Affymetrix Drug Metabolism Enzymes and Transporters (DMET) Microarray system. In this example, simultaneous genotyping of a large number of known markers (1,936 markers in 225 genes) was carried out. Earlier work validated the use of saliva on the Illumina Hap370 Microarray technology [60], as well as across two genotyping platforms (the Applied Biosystems Taqman™ and Illumina BeadChip™ genome-wide arrays [61], so it is hoped that over the course of the next few years, some of these and the many other research applications published translate into future clinically relevant tests.

The advent of point-of-care devices for nucleic acid testing (POCMDx) from companies such as TwistDx, Biohelix, Rheonix, Douglas Scientific, Alere, and others could also offer up new opportunities for oral testing in the future. Currently these devices are based upon blood sampling technologies and would clearly benefit from validated noninvasive protocols using saliva.