Through most of the pandemic, the world has largely been reliant on nasal testing for diagnosing COVID infections. But the Omicron wave has rekindled the debate among experts over whether saliva tests are better or worse than nasal ones.
Recent studies suggest that with the Omicron variant, saliva swabs have a higher positive percentage agreement (PPA) than nasal swabs. In the case of the Delta variant, this trend was reversed.
Saliva tests may also reveal the presence of SARS-CoV-2 days before a nasal swab would, due to the virus’ early presence in the mouth. A respiratory virus expert from the University of Maryland, Dr Milton says “the virus shows up first in your mouth and throat, [meaning] the approach we’re taking to testing has problems.”
Dr Milton’s team recently reported saliva samples contained thrice the viral load of nasal samples, in the three days before symptom appearance and the two days after. In this period, saliva samples were twelve times as likely to yield a positive PCR test. However, three or more days after symptoms appeared, larger viral loads accumulated in the nose.
Some experts say Omicron’s heightened detectability in saliva swabs may be because it appears to replicate faster in the upper respiratory tract, with a shorter incubation period. Others hypothesise Omicron might replicate more successfully in mouth and throat cells than previous variants.
There are some cons to saliva testing:
The mouth is a far less controlled environment than the nose, with more factors that may affect the accuracy of a COVID test, such as easily affected pH.
Especially when people are sick or dehydrated, saliva becomes viscous and difficult to work with in labs. Also, collection methods for saliva tests, like spitting or coughing, pose a higher transmission risk than nasal swabs.
Unlike for nasal swabs, there is no standardised collection and analysis method for saliva tests. This is a major reason many studies on saliva tests have been inconsistent in their results.
Some experts say new studies promoting saliva tests should be received warily, as nasopharyngeal tests have ‘years and years’ of research suggesting they are superior in detecting respiratory viruses.
Nonetheless, saliva testing is less invasive than nasal swabs, can be reliably self-collected, and is more affordable for frequent testing. Supply chains for saliva tests are easier to establish and maintain.
While nasopharyngeal swabbing detects historical cases of COVID, saliva may be better for determining active cases. One study states, “testing saliva throughout infection could permit shorter isolation, allowing essential workers to return to their duties sooner.”
As countries around the world struggle to adapt to the overwhelming surge of Omicron, the benefits of saliva testing pose an interesting strategic avenue.
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