In a recent study, a more effective saliva test was developed and tested for COVID-19.
Rapid and accurate detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is essential for the control of the COVID-19 pandemic. Currently, nasopharyngeal swabs (NPS) and oropharyngeal swabs (OPS) are the upper respiratory tract specimens recommended for COVID-19 diagnostic testing. However, collecting nasopharyngeal swab samples to test for COVID-19 is challenging as it puts healthcare workers at risk of exposure to SARS-CoV-2.
Alternatively, saliva is a potential clinical specimen that could be used for COVID-19 testing. True saliva is a clear liquid that naturally collects in the mouth. Saliva samples are easier to collect than nasopharyngeal swabs. The problem with using saliva from patients is that they can become mixed with mucus or blood, making it difficult to process in the laboratory. The clinical performance of saliva and nasopharyngeal samples have been evaluated in various studies with conflicting results.
An additional processing step makes saliva samples more sensitive than nasopharyngeal swab samples
Research published in The Journal of Molecular Diagnostics added a simple processing step to saliva samples before testing. This extra sample processing step improved the rate of COVID-19 detection, removed the challenges of nasopharyngeal testing, and made it possible for use in SARS-CoV-2 mass surveillance.
Saliva was first collected from different settings, including hospitals, nursing homes, and drive-throughs. Saliva samples were processed using an Omni bead mill homogenizer before being tested for COVID-19 through real-time PCR (RT-PCR). Typical saliva samples are less sensitive compared to nasopharyngeal samples. However, the homogenizer improved the sensitivity of saliva samples so that they showed higher sensitivity than NPS samples.
Lead investigator Dr Ravindra Kolhe explained, “Saliva as a sample type for COVID-19 testing was a game changer in our fight against the pandemic. It helped us with increased compliance from the population for testing along with decreased exposure risk to the healthcare workers during the collection process.”
Higher COVID-19 detection rate in saliva compared to nasopharyngeal samples
In Phase I of the study, 240 sample pairs of NPS and saliva samples were tested for SARS-CoV-2 RNA by RT-PCR. It was found that 28.3% of NPS and/or saliva samples tested positive for SARS-CoV-2. The results showed that the detection rate was lower in saliva (50%) compared to NPS (89.7%). This was unsurprising since the saliva samples had not been processed through a bead mill homogenizer.
In Phase II, 189 matched sample pairs were processed and tested. It was found that 50.2% of NPS and/or saliva samples tested positive for SARS-CoV-2. The detection rate was higher in saliva (97.8%) compared to NPS samples (78.9%). Dr Kolhe suggested that the increased sensitivity of saliva samples may be due to the fact that the viscous gel-like consistency of saliva samples developed into a better consistency through the addition of the homogenization step.
Phase III involved five-sample pooling to assess the use of saliva for mass surveillance. The pooled testing results indicated a positive percentage agreement of 95% (19 out of 20 pools showing positive COVID-19 results). The negative percentage agreement was discovered to be 100%. Pooled testing will be crucial for SARS-CoV-2 mass surveillance due to the rapidly evolving rate of COVID-19 and the reopening of schools, travel, tourism, and social activities.
A limitation of this study is that homogenization adds an extra step in the workflow and will increase the processing time compared to NPS samples. However, there is a significant clinical benefit to using saliva samples for COVID-19 testing, either individually or as pooled samples.
Dr Kolhe concluded, “Monitoring SARS-CoV-2 will remain a public health need. The use of a non-invasive collection method and easily accessible sample such as saliva will enhance screening and surveillance activities and bypass the need for sterile swabs, expensive transport media, and exposure risk, and even the need for skilled healthcare workers for sample collection.”
Sahajpal N, et al. (2021). Clinical validation of a sensitive test for saliva collected in healthcare and community settings with pooling utility for severe acute respiratory syndrome coronavirus 2 mass surveillance. The Journal of Molecular Diagnostics. Retrieved from: https://www.jmdjournal.org/article/S1525-1578(21)00114-8/fulltext
Saliva can be more effective than nasopharyngeal swabs for COVID-19 testing (2021). EurekAlert! Retrieved from: https://www.eurekalert.org/pub_releases/2021-06/e-scb061021.php
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