Saliva samples may be reliable alternative to nasal swabs for SARS-CoV-2 testing

The gold standard for the diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is currently based on detection using quantitative reverse-transcription polymerase chain reaction (RT-qPCR) in nasopharyngeal swabs collected from individuals. There are multiple RT-qPCR methods that have been proven very accurate, sensitive, and reliable and nasopharyngeal swabs are optimal for detection of infection especially in the early stages.

However, in the latter stages of the disease, there is a decline in the viral load in the nasopharynx, and the virus often becomes detectable only in specimens taken from bronchoalveolar lavage or sputum. Hence, additional testing strategies are needed to broaden and increase testing opportunities. To overcome these limitations in mass screening for detection of SARS-CoV-2, saliva has been considered as an alternate specimen. While nasopharyngeal swab collection requires trained health care personnel, saliva collection is relatively easier, less invasive, creates less discomfort to the patient, and can even be done through self-collection. Saliva specimens would also be particularly advantageous while testing children.

Evaluating an easy and safe saliva sampling strategy in adults and children

A team of researchers from various institutes in Switzerland recently sought to develop and evaluate a strategy for saliva sampling that enables easy and safe sampling of virus-containing specimen in adults and children, with the possibility for home collection and straight forward processing in the lab. Their study has been published on the preprint server, medRxiv*.

In their paper, the researchers report the outcome of a head-to-head comparison of SARS-CoV-2 detection in saliva and nasopharyngeal swabs using RT-PCR in 1,187 adults and children at outpatient test centers and an emergency unit.

RT-PCR results in saliva and nasopharyngeal swabs showed 98% agreement

According to the results, a total of 252 individuals tested positive for SARS-CoV-2 in either nasopharyngeal swab or saliva specimens. SARS-CoV-2 RT PCR results in both the specimens showed a high level of agreement with an overall percent agreement of 98.0%. Despite lower viral loads in saliva specimens, sensitive detection of SARS-CoV-2 was observed in saliva up to a threshold of Ct 33 in the corresponding nasopharyngeal swabs. In patients with Ct over 33 in nasopharyngeal swab, the agreement rate declined, but still was at 55.9%.

Despite lower viral loads in saliva, we observed sensitive detection of SARS-CoV-2 in saliva up to a threshold of Ct 33 in the corresponding NPS (Positive Percent Agreement = 97.7%).”

Saliva can be safely and conveniently used as a reliable specimen for SARS-CoV-2 detection

This large study conducted a parallel analysis of nasopharyngeal swab and saliva specimens, and the results established that saliva is a reliable specimen for SARS-CoV-2 detection. For the researchers, this means it can be added to the diagnostic portfolio to increase testing in a more convenient manner in adults as well as children.

Our study demonstrates an excellent agreement of saliva in the head-to-head comparison with NPS and thus recommends saliva as alternate material for SARS-CoV-2 detection by RT-PCR.”

According to the authors, since the majority of the virus in the saliva is not locally produced and is likely secreted from infected cells in the nasopharynx, it is important to collect the material from the posterior oropharynx. This is also highlighted in the collection protocols in their study.

To summarize, the analysis by the researchers rated saliva as a valid alternate specimen option for SARS-CoV-2 detection using RT-PCR. Saliva collection is non-invasive and hence reduces discomfort to the patient during sample collection. It eliminates the need for trained staff, allows sample collection anywhere, and enables self-collection. More importantly, the results show that using saliva as the test specimen does not require adjustments in the RT-PCR diagnostics tests.

All these benefits, combined with high reliability in SARS-CoV-2 detection, as demonstrated in this head-to-head comparison with nasopharyngeal swabs, highlight the need to increase testing efforts using saliva specimens and monitoring SARS-CoV-2 infection more effectively.

Combined with the high reliability in detecting SARS-CoV-2 infection as demonstrated in our head-to-head comparison with the standard NPS, increasing and facilitating test efforts by monitoring SARS-CoV-2 infection in saliva is rapidly attainable and needs to be considered.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Susha Cheriyedath

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.

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