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 Table of Contents  
Year : 2021  |  Volume : 13  |  Issue : 4  |  Page : 294-297

Saliva as a reliable diagnostic tool during the coronavirus disease times: A focused review

1 Department of Orthodontics, SIBAR Institute of Dental Sciences, Guntur, Andhra Pradesh, India
2 NRI Academy of Medical Sciences, Guntur, Andhra Pradesh, India
3 Department of Oral and Maxillofacial Surgeon, Bharathi's Ora Care, Yelahanka, Bangalore, India

Date of Submission08-Dec-2020
Date of Decision19-Dec-2020
Date of Acceptance20-Jan-2021
Date of Web Publication08-Oct-2021

Correspondence Address:
K Raghu Nandan
Private Practioner, Bharathi's Ora Care, No. 737 & 707, Yelahanka New Town, Bengaluru - 560 064, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/IJDS.IJDS_206_20

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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also currently known as CoV disease 2019 (COVID-19), has created a global health upheaval. The saliva of an infected person serves as a source for the transmission of CoV. The angiotensin-converting enzyme-2 receptors serve as the host receptor cells for CoV which is expressed in high numbers in salivary glands, oral mucosa, and gingiva. The salivary gland is a potential reservoir for COVID-19 even in asymptomatic but infected carriers. This knowledge could be used to employ salivary sampling as a noninvasive diagnostic method. IgA, IgM antibodies detected in self-collected saliva show more stability than RNA and may aid in the identification of asymptomatic patients. Saliva shows high sensitivity and specificity in the diagnosis of COVID-19 with >90% concordance reported between the saliva and nasopharyngeal swabs. Standardization with respect to sample collection, storage, and transport media can help in validation of diagnostic application of salivary-based testing.

Keywords: Corona virus, reverse transcription-polymerase chain reaction, saliva and coronavirus disease-2019, salivary diagnostics, severe acute respiratory syndrome coronavirus 2

How to cite this article:
Shaukari S, Bellam S, Nandan K R, Peddu R. Saliva as a reliable diagnostic tool during the coronavirus disease times: A focused review. Indian J Dent Sci 2021;13:294-7

How to cite this URL:
Shaukari S, Bellam S, Nandan K R, Peddu R. Saliva as a reliable diagnostic tool during the coronavirus disease times: A focused review. Indian J Dent Sci [serial online] 2021 [cited 2021 Oct 19];13:294-7. Available from: http://www.ijds.in/text.asp?2021/13/4/294/327814

  Introduction and Background Top

Declared as a pandemic by the WHO on March 11, 2020, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), also currently known as CoV disease-2019, (COVID-19) has created a global health upheaval.[1]

The last two decades have witnessed three major global health issues, namely the SARS CoV, Middle East Respiratory Syndrome (MERS) CoV, and the SARS-CoV-2, with a mortality rate of 9.5%, 34.4%, and 2.3%, respectively. Although COVID-19 expresses a much lower mortality rate, it has raised alarm with its capability to spread much more rapidly than MERS-CoV and SARS-CoV.[2]

  Transmission Top

Majorly, COVID-19 transmission occurs through the direct and indirect modes. Direct mode of transmission occurs through respiratory droplet nuclei generated by an infected person while talking, coughing, sneezing, and through aerosols generated during the clinical procedures. Body fluids and discharges, for example, feces, saliva, urine, semen, and tears also serve as a potential transmission route. The exposure and risk of transmission is elevated if the infected person is present within 1 m radius of a susceptible host. Indirect transmission may occur through fomites or through surfaces, objects that have come in contact with the infected.[3]

  Tests for Detecting Coronavirus Disease-2019 Top

Most of the tests used for Covid-19 detection can be grouped under the polymerase chain reaction (PCR) or serological tests. All these necessitate sample collection employing various means to identify the characteristic hallmarks of the SARS-CoV-2 virus.[4]

Although chest computed tomography scans sound promising with high sensitivity for COVID-19 detection, they cannot be employed for mass screening due to low specificity, huge machinery involved, difficulty in sterilization as well as increased radiation exposure.[5]

Reverse transcription-PCR is being considered as the gold standard for the detection of novel CoV.[6] However, this requires the need for nasopharyngeal swabbing by skilled medical personnel. In addition, it is known to be less tolerant to patients. Literature documents it to be contraindicated in patients with significant nasal septum deviation, recent facial trauma, patients on anti-coagulant therapy and thrombocytopenic patients; in whom bleeding can be anticipated. Furthermore, nasopharyngeal swabs are not considered to be suitable for serial controlling of viral load.[6]

All these limitations clearly picturize the need for a simpler sample collection technique which is not only less invasive but with minimal logistic burden.[7],[8]

  Saliva and its Potential Role in Coronavirus Disease-2019 Diagnosis Top

Sputum is a noninvasive lower respiratory tract specimen which has immense diagnostic value in diseases such as tuberculosis and chronic obstructive pulmonary disease, but Huang et al.[9] in their study stated that only 28% of patients with 2019-nCoV can produce sputum. It also represents a drawback that it is mandatory to collect sputum only before tooth brushing and breakfast, as nasopharyngeal secretions move posteriorly, and bronchopulmonary secretions move by ciliary activity to the posterior oropharyngeal area, whereas the patients are in a supine position during sleep.[10] Hence, these limitations eliminate sputum as a reliable diagnostic aid in COVID-19.

Few others have suggested the employment of blood specimens for rapid PCR-based assays or immunochromatography-based in vitro assays to detect specific antibodies. Although these techniques sound promising in terms of time, their suitability for screening at the large scale is questioned by procurement of blood samples at a medical point-of-care.[10]

The saliva of an infected person serves as a source for CoV[3],[6] and this knowledge could be further investigated to determine if a noninvasive salivary diagnostic method could be employed for the COVID-19 detection for limiting the spread.[11]

Asymptomatic patients provide a major challenge in the current diagnosis protocols of many countries. In a study by Azzi et al.,[10] two patients who were monitored showed positive salivary results on the same days that their pharyngeal or bronchoalveolar swabs were negative. This raises the possibility that individuals can be contagious through their saliva even when pharyngeal swabs are negative. This could be a point in favor of employing saliva for virus detection and also further strengthens the view that salivary glands serve as potential reservoir for Covid-19 virus in asymptomatic but infected people.

Diagnostic strategies with high specificity and sensitivity for both asymptomatic and symptomatic potential “super spreaders” may help combat against this cataclysmic pandemic.[12]

Different pathways have been put forward to substantiate the presence of COVID-19 in saliva: First, it might be present in the liquid droplets constantly being exchanged in the upper and lower respiratory tract that enters the oral cavity. Second, virus in the blood can reach the oral niche through crevicular fluid. A study of SARS CoV in Rhesus macaques widely documented the susceptibility of epithelial linings of the salivary glands to infection. From the same, it might be suggested that infections of the minor, major salivary glands, and its ducts can serve as an entry portal for COVID-19. Drainage of debris from nasopharyngeal epithelium into the oral cavity serves as another potential route.[8],[11]

Xu et al.[13] have demonstrated that the receptor-binding domain of SARS-CoV-2 spike protein supports strong interactions with the human angiotensin-converting enzyme-2 (ACE-2) receptor seen in taste cells, which might be responsible for infection-related ageusia. ACE-2 protein is also expressed in vascular endothelial cells, alveolar epithelial cells of lungs, enterocytes of the intestine, and myocardial cells. Hence, justifying the higher susceptibility of these organs to get infected by SARS-CoV-2.[14] Appearance of ACE-2 is higher in minor salivary glands than that in the lungs.[7] Among other oral sites, epithelial cells of the tongue exhibit the highest expression of ACE-2 followed by the buccal tissues and gingiva. Endothelial cells of the oral mucosa show exaggerated expression of ACE-2 during SARS-CoV-2 infection and may be responsible for the high viral load in saliva.[15]

Brann et al.[16] interpreted that the COVID-19 virus has damaging effects on nonneuronal olfactory epithelium causing anosmia, and its intimate correlation to the chemosensory senses of taste may be responsible for the resultant ageusia.

Investigations have further confirmed that SARS-CoV RNA is present in the saliva before the emergence of lesions in the lung. The positive rate of COVID-19 in salivary samples can exceed about 92%, and live virus can also be cultivated through the salivary samples.[7] Whereas certain scenarios have required repeated nasopharyngeal swab sampling before a positive finding has been obtained in otherwise overt COVID-19 patients, suggestive of its compromised reliability.[10] Many authors have substantiated the promising role of saliva in COVID-19 detection with high sensitivity, specificity, and consistency[10],[15]. More than 90% concordance has been displayed between self-collected saliva and nasopharyngeal swabs collected by health-care workers.[15]

Various methods have been mentioned in the literature to collect saliva. A simple drooling technique has been suggested by Azzi et al.[10] to collect saliva and the collected specimen was suspended in 2 ml of phosphate-buffered saline. To et al.[17] collected saliva by coughing up and clearing the throat such that the collected sample contained fluid from the posterior oropharynx also. Williams et al.[18] suggested a method in which prior to collection, patients were asked to pool saliva and then spit 1–2 ml into a collection tube. However, this method of saliva pooling in the mouth might lead to dilution of the viral load in the specimen because of excessive saliva secretion.

  Timing of Sampling Top

The fact that saliva may depict alteration due to physiological processes taking place throughout the day must be taken into consideration. Most of studies have requested participants to avoid oral stimulation such as eating, drinking, and oral hygiene practices during a certain period before a sample is taken therefore establishing standard sample conditions due to the presence of circadian rhythms in saliva and their effect on its composition.[8],[19]

It is recommended to collect saliva between 8 and 10 a. m. to reduce the influence of circadian rhythms on the results.[19] Tajima et al.[20] also found that early morning saliva specimens were more likely to show positive 2019-nCoV RNA results than daytime saliva specimens. The positive rate of saliva specimens collected during the early phase of onset defined as within 9 days of symptom onset was 93.4% and 63.0% after 10 days of symptoms.[20],[21]

  Prospect of Salivary Biomarkers in Coronavirus Disease-2019 Prognosis Top

Coronaviruses delays or decreases interferon (INF) production which activates inflammatory responses triggering severe pulmonary conditions.[22],[23] The unregulated host immunological response and the “cytokine storm” are considered to correlate with poor prognosis and severity of the disease.[22]

Recent studies have outlined that severity of COVID-19 cases can be correlated with elevated levels of granulocyte colony-stimulating factor, interleukin (IL)-2, IL-6, IL-7, IL-10, INF-γ-inducible protein-10, macrophage inflammatory protein-1A, tumor necrosis factor-α, and macrophage chemotactic protein, suggestive of inflammatory response mediated by cytokine release having a pivotal role in COVID-19 progression.[9],[23]

Saliva provides an opportunity to measure the markers of the inflammatory process such as chemokines and cytokines. Thus, evaluation of inflammation-related biomarkers in saliva can provide us with a platform to establish inflammatory profile assisting in prognosis stratification of COVID-19-infected patient.[24]

  Antibodies against Severe Acute Respiratory Syndrome Coronavirus 2 Top

Supplementary to RT-PCR-based RNA detection of COVID-19, few have documented encouraging results for the detection of IgG and IgM antibodies in the serum/plasma samples of infected patients. Further investigations suggest it may be justified to hypothesize that human saliva is a source for anti-SARS-CoV-2 antibodies.[24]

Antibodies such as IgA and IgM have been detected in different biological fluids including self-collected saliva. These being more stable than RNA simplify the sampling process and aid in the identification of asymptomatic patients. Antibody detection tests also assist to track down how competitively the patient's immune system is fighting against the virus and are potential guide for plasma transfusion therapies.[24],[25]

RUCDR Infinite Biologics at Rutgers University have stated that salivary testing equals in performance to the approved swab-based collection technique.[26]

The salivary diagnostic technique offers us with an additional advantage of:[27]

  • Real-time diagnostic value
  • Any number of samples can be easily collected
  • Eliminates the need for specific clinical setup for the collection and screening of samples
  • Unlike blood, saliva does not clot
  • In comparison to serum, saliva requires less manipulation during the diagnostic procedures.

  Conclusion Top

Salivary sampling for COVID-19 detection ensures high sensitivity, efficacy, reliability specificity, and concordance. Saliva offers a promising diagnostic alternative and can minimize the risk of nosocomial transmission. The current article highlights the pivotal role of saliva in COVID-19 detection.

Scope for future research

Insight into diagnostic application of saliva for COVID-19 detection brings with it a strong promise, at the same times opens up immense scope for investigation with regard to optimal temperature for sample collection, storage, and ideal transport media. Standardization of an appropriate assay can help in validation for the diagnostic application of saliva. The reliability of gingival crevicular fluid as a diagnostic source for COVID detection also needs further insight.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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