Saliva-based tests could go a long way in relieving the supplies shortages that have hampered COVID-19 testing efforts. The question, though, is whether saliva samples yield results as accurate as those produced by respiratory samples obtained by nasal and nasopharyngeal (NP) swabs. The good news is that a new
study suggests that at least one of these saliva-based tests is every bit as reliable as the tests based on samples obtained by swabs.
The Diagnostic Challenge
Real-time reverse transcription polymerase chain reaction (RT PCR) testing for qualitative detection of SARS-CoV-2 nucleic acid is the current gold standard for accuracy. The downside of RT PCR testing is its cost and lack of scalability, due in part to its reliance on upper and lower respiratory specimens that must be collected via NP swabs or bronchoalveolar lavage. In addition to being uncomfortable for patients, the sample collection process must be performed by qualified health care professionals using long cotton swabs, both of which happen to be in short supply right now. The process also sucks up scarce respiratory masks and other personal protective equipment (PPE) to the that it directly exposes the health professional to risk of infection.
The Advent of Saliva-Based Tests
Saliva-based testing overcomes all these bottlenecks. At least half a dozen saliva-based SARS-CoV-2 tests have received Emergency Use Authorization (EUA) the U.S. Food and Drug Administration (FDA). The first EUA for a saliva test was granted on April 13 to the Rutgers Clinical Genomics Laboratory TaqPath SARS-CoV-2 Assay test is based on the Thermo Fisher Scientific Applied Biosystems TaqPath COVID-19 Combo Kit which had been previously approved for detection in NP swab, nasopharyngeal aspirate and bronchoalveolar lavage specimens.
The Study
The simplification of saliva sample collection and elimination of the health professional “middleman” naturally raises questions about test results reliability. The new study, which was published in the
New England Journal of Medicine on Aug. 28, evaluates the accuracy of one of the newfangled saliva-based test products, the Yale School of Public Health’s SalivaDirect test for qualitative detection of nucleic acid from SARS-CoV-2 in saliva collected without preservatives in a sterile container.
The researchers studied a total of 70 inpatients with confirmed cases of COVID-19. After COVID-19 was confirmed with a positive NP swab at hospital admission, saliva specimens were collected by the patients themselves. Another NP test was done at the same time and health care workers collected and tested both the swabs and the saliva specimens.
Using primer sequences from the U.S. Centers for Disease Control and Prevention (CDC), the researchers found more SARS-CoV-2 RNA copies in the saliva specimens than in the NP swab specimens. At one to five days after diagnosis 81 percent of the saliva samples tested positive, compared to 71 percent for the NP swab samples. Additionally, a higher percentage of saliva specimens tested positive than the NP swab specimens tested positive 10 days after diagnosis. However, after 11 days only 41 percent of the saliva collected samples tested positive versus 50 percent of the NP swab collected samples. Based on these findings, the study authors concluded that saliva specimens and nasopharyngeal swab specimens have at least similar sensitivity in the detection of SARS-CoV-2, the authors of the study concluded.
Takeaway
The study supporting the reliability of the Yale saliva test bodes well but will, of course, need to be bolstered by further studies and evaluations of other products. Currently, at least half a dozen saliva-based SARS-CoV-2 tests have received EUA from the FDA, including products from Fluidigm, DxTerity Diagnostics, Clinical Reference Laboratory, P23 Labs and the Yale School of Public Health. With many more test makers having saliva-based products in the pipeline, that number is likely to at least double by the end of the year. 