Sir, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) is the most commonly used molecular diagnostic test for the detection of COVID-19 in biological samples but no universally accepted test is currently available, with several countries adopting different test strategies.1 The selection of proper location/test site for sample collection is very important to obtain reliable test results, the most commonly used being naso- and/or oropharynx swabs (NOS).2
Although these are relatively easy to collect and test results are highly sensitive, there are limitations related to sample collection and healthcare personnel safety. However, use of saliva as an alternative to NOS for detection of COVID-19 has been suggested.3,4 Using saliva samples has a number of 'clinical advantages'; it is less invasive and more convenient to patients as compared to NOS or blood samples4(especially desirable in multiple testing for disease monitoring). Secondly, with clear instructions, patients can collect saliva themselves, thereby minimising the risk of virus transmission to healthcare personnel and avoiding use of personal protective equipment.
This potential use of saliva seems scientifically reasonable as it has been shown to contain live COVID-19 viruses3 possibly containing a pool coming from the lower respiratory tract, nasopharynx and infected salivary glands (for some of the coronaviruses, infection of salivary glands occurs very early in the disease process).4 Unlike the other SARS virus diseases, the content of salivary COVID-19 (viral load) has been shown to be highest during the first week after symptom onset.5 This emphasises the role of saliva as a potential source of viral transmission and, as it could be detected in the saliva as long as 25 days after the onset of symptoms, suggesting its potential use for monitoring viral clearance.5
Besides RT-PCR-based tests, serological (enzyme immunoassays) diagnostic tests aimed at detecting IgM and IgG antibodies against the viral antigens are robust methods to diagnose current and past infections. Preliminary studies using serum samples have shown promising results for COVID-19.5,6 The production of SARS-CoV-specific secretory IgA in the saliva of animal models was previously shown.7 Hence, it is reasonable to speculate that anti-COVID-19 antibodies might also be present in human saliva although this has not been reported nor has their potential use for diagnostics and disease monitoring. This clearly warrants future studies.
Studies are needed to analyse the sensitivity and specificity of saliva based COVID-19 tests before they could be made available as a convenient and cost-effective diagnostic method. Since the presence of live COVID-19 in saliva identifies it as a potential source of viral transmission any collected saliva samples must be handled with care to avoid spilling and spreading of live viruses.
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Sapkota, D., Thapa, S., Hasséus, B. et al. Saliva testing for COVID-19?. Br Dent J 228, 658–659 (2020). https://doi.org/10.1038/s41415-020-1594-7
Journal of Clinical Medicine (2020)