SARS-CoV-2 Saliva Testing Gains Traction

Saliva-based SARS-CoV-2 tests emerged as an alternative COVID-19 diagnostic early during the COVID-19 public health emergency (PHE) when the nasopharyngeal swabs used to collect tissue samples for conventional tests were in desperately short supply. While saliva test kits and products received FDA Emergency Use Authorization (EUA) during the crisis, saliva testing has had only a marginal impact. But that may be about to change now that the agency has, for the first time, cleared a SARS-CoV-2 saliva test for over-the-counter use.

The Advantages of Saliva Testing

The principal method of detecting the SARS-CoV-2 virus is to perform polymerase chain reaction (PCR) tests capable of detecting RNA from the virus. PCR tests are high in specificity and sensitivity. But they must be performed on tissue samples collected from the back of the sinus cavity by inserting a long nasopharyngeal swab deep into the nostril. There are certain disadvantages to this method:

• It is uncomfortable for patients and may contribute to “testing fatigue”
• It may not be suitable for young people or those with special needs
• It requires the use of long swabs that may be in short supply
• Samples must be collected by trained staff wearing personal protective equipment (PPE), both of which are subject to the risk of shortage

Saliva-based testing eliminates these disadvantages. It is noninvasive, suitable for all individuals, simple, and largely impervious to supply chain disruptions. Saliva samples are also stable and can be preserved for at least 20 days at 39.2°F/4°C without viral transport.

The Accuracy Issue

Given these advantages, why has saliva-based SARS-CoV-2 testing not become more widespread? A big part of the answer is concerns over test reliability, specifically the lack of data supporting saliva testing accuracy. Thus, about one year into the pandemic, the influential Infectious Diseases Society of America issued guidelines stating that “saliva as the sole sample source of COVID-19 diagnostics cannot be recommended due to a paucity of studies.”¹

However, even before the pandemic began, there was growth in the research demonstrating the sensitivity and specificity of saliva testing vis-à-vis swab testing.

For example, an article published in the online version of Lancet Respiratory Medicine in April 2021 suggests that by November 2020, there were 58 studies evaluating SARS-CoV-2 detection in saliva—54 using RT-PCR, four using reverse transcription loop-mediated isothermal amplification, and three using both.² However, because of the differences in methodologies used by the studies, detection rates were highly variable. Thus, of the RT-PCR studies:

• Most (69 percent) found greater or similar (≤10 percent difference) sensitivity between saliva and nasopharyngeal and other swab methods;
• Only 24 percent found saliva to be less sensitive than swabs; and
• Seven percent showed that results varied based on population group, stage of infection, or method.

The overall implication is that saliva-based detection is at least similar in sensitivity to swab-based detection. However, “using less robust methods can skew the sensitivities reported across studies,” the authors noted. “Importantly, saliva detected an additional 10 percent of positive individuals for whom swabs produced false negative results.”

Development of Saliva SARS-CoV-2 Tests

The laboratory industry has developed saliva-based COVID-19 diagnostics products. In April 2020, less than three months into the PHE, Rutgers University secured the first EUA for a saliva COVID-19 test, the Rutgers Clinical Genomics Laboratory TaqPath SARS-CoV-2 Assay test which runs on the Thermo Fisher Applied Biosystems QuantStudio 5 Real-Time PCR System.⁵ A month later, Rutgers’ RUCDR Infinite Biologics received an expanded EUA for a saliva assay performed on samples that users collected at home and sent to a laboratory for testing. The test, developed by RUCDR in partnership with Spectrum Solutions and Accurate Diagnostic Labs, became the first saliva assay to not require any physical interaction with a healthcare professional.

As of the date of publication, 35 different saliva-based kits have received EUA during the PHE, many for use at the point of care. That includes products from not only academic institutions but also leading commercial laboratory companies, including Thermo Fisher Scientific, PerkinElmer, BioMérieux subsidiary BioFire, Ambry Genetics, Rheonix, and NeuMoDx Molecular.

On Oct. 18, the FDA broke some new ground by issuing the first EUA for an over-the-counter saliva-based COVID molecular test to Aptitude Medical for the Metrix COVID-19 Test, a single-use test that uses real-time loop-mediated isothermal amplification (RT-LAMP) with a fluorescence reader to detect the SARS-CoV-2 virus.

SARS-CoV-2 Saliva Diagnostic Products Cleared by FDA (as of November 10, 2022)

References:

1. https://www.idsociety.org/practice-guideline/covid-19-guideline-diagnostics/
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8055204/
3. https://www.nejm.org/doi/pdf/10.1056/NEJMc2016359?articleTools=true
4. https://www.acpjournals.org/doi/10.7326/M20-4738
5. https://www.rutgers.edu/news/new-rutgers-saliva-test-coronavirus-gets-fda-approval