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Saliva specimens for SARS-COV-2 detection: 2 studies show parallel positivity and sensitivity

2020-09-01
SARS-COV-2 EM photo courtesy NIAID

A letter to the editor in the New England Journal of Medicine on August 28 describes higher virus counts in saliva specimens than in nasopharyngeal swab specimens taken from the same patients.

From the letter:

We therefore screened 495 asymptomatic health care workers who provided written informed consent to participate in our prospective study, and we used RT-qPCR to test both saliva and nasopharyngeal samples obtained from these persons. We detected SARS-CoV-2 RNA in saliva specimens obtained from 13 persons who did not report any symptoms at or before the time of sample collection. Of these 13 health care workers, 9 had collected matched nasopharyngeal swab specimens by themselves on the same day, and 7 of these specimens tested negative …. The diagnosis in the 13 health care workers with positive saliva specimens was later confirmed in diagnostic testing of additional nasopharyngeal samples by a CLIA (Clinical Laboratory Improvement Amendments of 1988)–certified laboratory.

https://www.nejm.org/doi/full/10.1056/NEJMc2016359

A letter to the Annals of Internal Medicine, also on August 28, found similarly discordant test results: (in this study both nasopharyngeal and oropharyngeal swabs were used in separate cases)

Of the 1939 paired swab and saliva samples analyzed (Figure), SARS-CoV-2 E gene was detected in 70 samples (Table), 80.0% with swabs and 68.6% with saliva. Thirty-four participants (48.6%) tested positive for SARS-CoV-2 on both swab and saliva samples. Discordant test results were seen in 22 participants (31.4%) who tested positive with swab alone and in 14 (20%) who tested positive with saliva alone. Swabs were obtained from the nasopharynx in 35.7% of participants who tested positive with saliva alone, compared with 9.1% of participants who tested positive with swab alone.

https://www.acpjournals.org/doi/10.7326/M20-4738

The second study used a preservative fluid for the saliva specimen to prevent degradation of the RNA during room-temperature storage and transport of the specimen: “This study is unique in that it used a novel collection kit containing a preservative and viricidal [sic] fluid, allowing for safe and stable storage and transport of the samples.”

The first study was much smaller than the second, so the finding of somewhat greater sensitivity for saliva specimens may have been a fluke. Nonetheless, it is clear that saliva specimens are similarly sensitive as nasopharyngeal specimens.

These two studies suggest that it may be useful to test patients who have a high index of suspicion for COVID-19 with both nasopharyngeal swabs and saliva tests. Nasopharyngeal swabs have a significant false negative testing rates, a factor which is not widely discussed. The discordance of positive tests with one type of specimen and negative results with the second type supports the idea of using both tests when one is highly suspicious of infection.

The studies indicate that saliva testing is a viable means for detecting SARS-COV-2 RNA. The saliva test has the advantage of dispensing with an invasive nasopharyngeal swab, eliminating one component of the test which may be in short supply. In addition, the saliva test does not require a technician for collection, eliminating the risk of exposure to infection by another person.

The only roadblock to using saliva tests is certification by the Food and Drug Administration for these tests. Emergency Use Authorization should be immediately given for saliva tests.

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