Saliva test for COVID-19 diagnosis and severity prediction - Dr. Amisha Parekh

Saliva has been considered as a potential pool for biomarkers in several diseases for years. Its importance in the diagnosis and prognosis of COVID-19 cases has increased multifold in the last twelve months after the pandemic outbreak.

Countries like South Korea, Germany, and Japan have implemented saliva testing for COVID-19 detection and in the USA, saliva testing is being used for the reopening of educational institutions [1]. Recently, a smartphone-read portable, ultrasensitive and quantitative saliva test for COVID-19 has been developed which has a 15-min sample-to-answer time that does not require RNA isolation or laboratory equipment [2].

Saliva for COVID-19 diagnosis: Recently several studies have reported the diagnostic potential of saliva for COVID-19 [3-10] and the following can be summarized

• SARS-COV-2 viral load in saliva has been found to be either higher or equivalent as compared to standard nasopharyngeal swab test and saliva is likely to be constantly positive throughout the course of infection.
• SARS-CoV-2 can be detected in the saliva of asymptomatic persons.
• Several preliminary reports showed that the viral load in the saliva is comparable with that in sputum and sputum is superior to nasopharyngeal swabs in the detection of SARS-CoV-2 infection.
• Overall median CT value in saliva samples was found to be higher than the nasopharyngeal swab.
• Anti-SARS-CoV-2 antibodies in saliva have been found to reflect concentrations in serum thus making saliva testing a good alternative to blood-based antibody testing.
• Saliva has been suggested to be preferable for gauging active rather than historical infection which could help in preventing unnecessarily prolonged isolation periods.
• Studies that used methods involving clear saliva (rather than sputum) and processing to reduce the viscosity (eg, homogenization) showed greater sensitivities relative to swabs. Methods requiring forceful production of saliva, such as by spitting or coughing instead of drooling, reduced sensitivity.

Salivary viral load correlates with COVID-19 severity and mortality [11]

• High salivary viral load was correlated with increasing levels of disease severity and showed a superior ability over nasopharyngeal viral load as a predictor of mortality over time. 
• High salivary viral load correlated with many known COVID-19 inflammatory markers such as IL-6, IL-18, IL-10, and type 1 immune response cytokines as well as with progressive depletion of platelets, lymphocytes, and effector T cell subsets including circulating follicular CD4 T cells. 
• The salivary viral load has been found to be significantly higher in those with COVID-19 risk factors such as older age, heart failure, cancer, and certain immunosuppression states, hypertension, and chronic lung disease as well as in males who have been known to be at a higher risk than females.
• High salivary viral loads required a higher antibody-to-virus ratio to successfully bring down viral load. Thus, monitoring salivary viral load could play a very significant role when considering the incorporation of convalescent serum in patients.

Conclusion

The Lancet states that “A single saliva sample can simultaneously enable the identification of active cases, previous cases, and vaccine-induced immune responses. Importantly, salivary antibody testing provides a scalable means for monitoring herd immunity in a post-vaccination world” [1].

A simple non-invasive saliva test could not only help in diagnosing COVID-19 cases but also help in evaluating the prognosis of patients. Mouth rinses like chlorhexidine have been known to reduce the viral load in the oropharyngeal region [12,13,14] and the use of such rinses in patients diagnosed with high salivary viral load could help in reducing transmission of the virus as well as may help in improving the prognosis of patients by reducing the salivary viral load. Thus, a simple saliva test is much needed in these critical times.

References

1. Lancet Respir Med 2021 Published Online April 19, 2021 https://doi.org/10.1016/ S2213-2600(21)00178-8.
2. Ning B, Yu T, Zhang S, et al. A smartphone-read ultrasensitive and quantitative saliva test for COVID-19. Sci Adv. 2021;7(2):eabe3703. Published 2021 Jan 8. doi:10.1126/sciadv.abe3703.
3. Byrne RL, Kay GA, Kontogianni K, et al. Saliva Alternative to Upper Respiratory Swabs for SARS-CoV-2 Diagnosis. Emerging Infectious Diseases. 2020;26(11):2769-2770. doi:10.3201/eid2611.203283.
4. Hung KF, Sun YC, Chen BH, et al. New COVID-19 saliva-based test: How good is it compared with the current nasopharyngeal or throat swab test?. J Chin Med Assoc. 2020;83(10):891-894. doi:10.1097/JCMA.0000000000000396.
5. Butler-Laporte G, Lawandi A, Schiller I, et al. Comparison of Saliva and Nasopharyngeal Swab Nucleic Acid Amplification Testing for Detection of SARS-CoV-2: A Systematic Review and Meta-analysis [published correction appears in doi: 10.1001/jamainternmed.2021.0245]. JAMA Intern Med. 2021;181(3):353-360. doi:10.1001/jamainternmed.2020.8876.
6. Babady NE, McMillen T, Jani K, et al. Performance of Severe Acute Respiratory Syndrome Coronavirus 2 Real-Time RT-PCR Tests on Oral Rinses and Saliva Samples. J Mol Diagn. 2021;23(1):3-9. doi:10.1016/j.jmoldx.2020.10.018.
7. Wyllie AL, Fournier J, Casanovas-Massana A, et al. Saliva or Nasopharyngeal Swab Specimens for Detection of SARS-CoV-2. N Engl J Med. 2020;383(13):1283-1286. doi:10.1056/NEJMc2016359.
8. Teo AKJ, Choudhury Y, Tan IB, et al. Saliva is more sensitive than nasopharyngeal or nasal swabs for diagnosis of asymptomatic and mild COVID-19 infection. Sci Rep. 2021;11(1):3134. Published 2021 Feb 4. doi:10.1038/s41598-021-82787-z.
9. Rao M, Rashid FA, Sabri FSAH, et al. COVID-19 screening test by using random oropharyngeal saliva. J Med Virol. 2021;93(4):2461-2466. doi:10.1002/jmv.26773.
10. Wyllie, Anne & Fournier, John & Casanovas-Massana, Arnau & Campbell, Melissa & Tokuyama, Maria & Vijayakumar, Pavithra & Geng, Bertie & Muenker, mary catherine & Moore, Adam & Vogels, Chantal & Petrone, Mary & Ott, Isabel & Lu, Peiwen & Venkataraman, Arvind & Lu-Culligan, Alice & Klein, Jonathan & Earnest, Rebecca & Simonov, Michael & Datta, Rupak & Ko, Albert. (2020). Saliva is more sensitive for SARS-CoV-2 detection in COVID-19 patients than nasopharyngeal swabs. 10.1101/2020.04.16.20067835.
11. Silva J, Lucas C, Sundaram M, et al. Saliva viral load is a dynamic unifying correlate of COVID-19 severity and mortality. Preprint. medRxiv. 2021;2021.01.04.21249236. Published 2021 Jan 10. doi:10.1101/2021.01.04.21249236.
12. Chitguppi, Rajeev. (2020). Chlorhexidine gluconate is effective against the novel coronavirus & other viruses. 10.13140/RG.2.2.18594.99524.
13. Chitguppi, Rajeev, Mouth Rinses with Substantivity Can Prevent COVID-19 Spread and Protect the Healthcare Workers (June 30, 2020). Available at SSRN: https://ssrn.com/abstract=3638601 or http://dx.doi.org/10.2139/ssrn.3638601.
14. Huang, YH, Huang, JT. Use of chlorhexidine to eradicate oropharyngeal SARS‐CoV‐2 in COVID‐19 patients. J Med Virol. 2021; 1‐ 4. https://doi.org/10.1002/jmv.26954.