Why Are You Scratching My Brain?  Rapid, Extraction Free SARS CoV-2 Testing from Saliva

Perhaps the most effective weapon in the fight against SARS CoV-2 will be the deployment of inexpensive, rapid diagnostic tests. The conventional testing requires the nasopharyngeal swabs taken as samples from patients or sample providers, followed by extraction of viral RNA and detection by RT-qPCR. This procedure is lengthy, offers low sensitivity during early infection and due to increase in demand and testing, the swabs and other raw materials required for viral nucleic acid extraction have become limited or nearly exhausted. Furthermore, the procedure of taking nasopharyngeal swab is invasive and may lead to coughing in some patients which increases the risk of nosocomial spread of virus.

The limitations of nasopharyngeal swabs have compelled the scientific community to look for alternatives diagnostic approaches including alternative samples for testing. Saliva based assays are emerging as promising alternative to the nasopharyngeal swab. Previous studies have indicated the sensitivity of saliva samples is comparable to nasopharyngeal swabs¹ and data released in early 2020 demonstrate the efficacy of a saliva based assay in the detection of COVID-19^2,3. Saliva based diagnostics offer advantages such as ease of collection, self-administration, and do not require swabs or a sterile collection tube.

Looking for an alternative and quick ways to process sample and detect Covid 19, FDA has recently approved under EUA (Emergency Use Authorization) the use of SalivaDirect Assay ^{4, 5}. In SalivaDirect assays(Fig.), saliva samples are collected from the sample provider and treated with proteinase K (SalivaOptimize™ Proteinase K) to digest enzymes and proteins. This is followed by inactivation of Proteinase K by heating the sample at 95°C. The saliva sample treated with proteinase K is directly used for RT-qPCR for COVID 19 detection⁵. The SalivaDirect Assay does not require any lengthy process of RNA extraction. Furthermore, the sensitivity of the assay is equal or superior to nasopharyngeal swabs and viral nucleic acid extraction⁵. The Yale School of Public health⁵ group carried out SARS CoV-2 detection in paired nasopharyngeal swabs and saliva samples collected from Covid-19 patients and asymptomatic healthcare workers at moderate-to-high risk of COVID 19 exposure. They found that using saliva for SARS CoV-2 detection is more sensitive and consistent than using nasopharyngeal swabs. The consistency of SalivaDirect testing is further useful in studying the course of infection during extended hospitalization. Although the study is very promising, it is preliminary, and more data needs to be collected to confirm these findings.

References:

1. Kim Y. -G. et al (2017). Comparison between saliva and nasopharyngeal swab specimens for detection of respiratory viruses by multiplex reverse transcription-PCR. Journal of Clinical Microbiology. 55 (1): 226–233
2. To K. K. -W. et al (2020). Consistent detection of 2019 novel coronavirus in saliva. Clinical Infectious Diseases. Brief Report. CID 2020.xx (XXXXXX).1.
3. Wang W.K., Chen S. Y., Liu I.J., et al (2004). Detection of SARS-associated coronavirus in throat wash and saliva in early diagnosis. Emerg Infect Dis 2004; 10:1213-9.
4. https://www.fda.gov/media/141192/download
5. Wyllie A. L., et al (2020). Saliva is more sensitive for SARS-Cov-2 detection in covid -19 patients than nasopharyngeal swabs. MedRxiv. https://doi.org/10.1101/2020.04.16.20067835doi.