Diagnostic potential of saliva for SARS-CoV-2
It has been reported that the angiotensin converting enzyme II (ACE2) is the host cell receptor to which the SARS-CoV-2 binds to gain entry into cells, same as SARS-CoV.9,17 Xu et al have demonstrated that the receptor binding domain of SARS-CoV-2 spike protein supports strong interactions with the human ACE2 receptor.18 The ACE2 protein is present in most organs of the human body, and is abundantly expressed in the vascular endothelial cells, heart, alveolar epithelial cells of lungs and enterocytes of the intestine.19 These findings indicate that these organs may potentially be at high risk for COVID-19 infection.20 Recently, RNA sequencing studies from The Cancer Genome Atlas database have identified that there is a high expression of the ACE2 receptors on the epithelial cells of oral mucosa.21 Among oral sites, the highest expression was seen in the epithelial cells of tongue, followed by buccal and gingival tissues. These findings may provide clues for further investigation of oral routes of infection, pathogenesis and detection of COVID-19.
Previous studies have demonstrated that salivary specimens have a higher than 90% concordance rate with nasopharyngeal specimens in the detection of respiratory viruses.22 In an initial pilot study by To et al.,23 SARS-CoV-2 was detected in the salivary specimens of 11 out of 12 patients with laboratory-confirmed COVID-19, and all 33 individuals who tested negative for nasopharyngeal specimens also tested negative for salivary specimens. In another recently published study, posterior oropharyngeal saliva samples were collected for 23 patients with laboratory-confirmed COVID-19 for nasopharyngeal specimens.24 Of these, 20 patients (87%) tested positive for SARS-CoV-2 in their saliva. Serial viral load was ascertained using reverse transcriptase quantitative polymerase chain reaction (RT-qPCR). It was found that the salivary viral load was highest during the first week after symptom onset and it declined over time. The salivary sample was self-collected by patients by coughing up saliva from the posterior oropharynx. Therefore it is possible that these specimens included secretions from the nasopharynx or the lower respiratory tract, rather than being completely salivary. In a study by Williams et al.,25 622 individuals were tested for COVID-19 using nasopharyngeal specimens, of which 522 also provided salivary specimens. For salivary specimen collection, they were instructed to pool saliva in their mouth for 1-2 minutes, and gently spit into a collection pot. 33 of the 39 patients (84.6%) who tested positive for nasopharyngeal specimens, also had SARS-CoV-2 detected in their saliva.25 A study of 44 COVID-19 inpatients noted a high correlation between nasopharyngeal and salivary samples, with higher viral titers in saliva.26 It was found that the salivary specimens yielded higher detection sensitivity and consistency throughout the course of disease. This study also enrolled 98 asymptomatic healthcare workers, who self-collected nasopharyngeal and salivary specimens every three days for a period of two weeks. SARS-CoV-2 was detected in saliva from two healthcare workers who tested negative for matching nasopharyngeal samples.26 These data, although limited, suggest that saliva may be more sensitive in detection of asymptomatic or pre-symptomatic infections.
Researchers from Rutgers University evaluated the use of salivary specimens for SARS-CoV-2 detection in symptomatic patients from three ambulatory care centers.27 They found 100% positive and negative concordance between results obtained from testing of saliva and those obtained from nasopharyngeal and oropharyngeal swabs [saliva vs nasopharyngeal swab: (26/26) positive agreement, (27/27) negative agreement; saliva vs oropharyngeal swab: (4/4) positive agreement, (3/3) negative agreement]. The U.S. Food and Drug Administration (FDA) has since issued them an emergency use authorization for the use of salivary specimens, in addition to the nasopharyngeal and oropharyngeal swabs, for detection of SARS-CoV-2 RNA in individuals suspected of COVID-19.27 A salivary diagnostic test would allow for a noninvasive self-administered sample collection under healthcare providers’ directive for individuals in quarantine, and would circumvent the issues regarding global shortage of swabs and personal protective equipment (PPE) needed for conventional COVID-19 testing. Specific guidelines are needed to standardize the method for collection of salivary specimens, and implement the use of appropriate assays, and processing methods. The presence of SARS-CoV-2 in the saliva of infected patients also bears implications for a high potential of transmission in the dental operatory, and underscores the need for awareness and use of effective PPE practices.