Regular testing has become more important as the Omicron variant of SARS-CoV-2, the virus that causes COVID-19, has continued to rise in many countries across the world. Scientists have compared different tests available in their ability to detect the variant, such as polymerase chain reaction tests and rapid antigen tests.
Now, scientists from South Africa revealed that saliva swabs are best suited to detect the Omicron variant compared to mid-turbinate swabs for the PCR tests. Meanwhile, US Food and Drug Administration (FDA) announced that rapid antigen tests are the worse in detecting the new variant.
A schoolgirl gives a sample during a saliva test for the COVID-19 at an elementary school in Eysines, on the outskirts of Bordeaux, southwestern France on February 25, 2021.
Saliva Swabs Can Detect More Viral RNA Than Nasal Swabs
According to News Medical Life Sciences, scientists used a composite standard for SARS-CoV-2 infection to compare saliva and nasal samples. Detection of viral RNA is considered a positive infection. Then they compared the two results to estimate their sensitivity and specificity in detecting the Omicron and Delta variants.
They observed that saliva and nasal swabs have a 71% and 100% detection rate for the Delta variant, respectively. On the other hand, saliva swabs displayed better detection of the Omicron variant at 100% compared to the 86% positive detection of the nasal swabs. They noted that the differences in RT-PCR cycle threshold between saliva and nasal swabs were estimated to be 5.2 for the Delta variant, while 1.5 for the Omicron variant.
Therefore, the findings showed that saliva swabs are more useful than nasal swabs in the accurate detection of the Omicron variant since saliva samples contain more viral RNA than nasal samples.
They explained that this might be due to the altered viral shedding patterns in the new variant, wherein studies reveal the Omicron variant replicates faster in the upper respiratory tract. It indicates that certain host tissues support the growth of the variant in a phenomenon called tissue tropism. Researchers recommend reassessing detection methods for optimal diagnosis.
ALSO READ: Omicron Multiplies 70X Faster in Airways, 10X Slower in Lungs Compared to Earlier COVID-19 Strains
Rapid Antigen Tests Worse in Detecting the Omicron Variant
The FDA said on Tuesday, December 28, that rapid antigen tests appear to be less sensitive to the Omicron variant than the previous variants. They said that early data suggests that although rapid antigen tests can still detect the new variant, they may also have reduced sensitivity.
According to Business Insider, this statement was based on laboratory findings which are the best way to evaluate the true test performance of tests in the short-term compared to the more dependable but longer clinical-trial results.
The news came out after the White House announced it would be distributing 500,000 free rapid antigen tests for COVID-19 and planned on making over-the-counter test kits free starting in January 2022. People still use rapid tests because they are cheaper and more convenient, but experts warn that the public should be aware of its shortcomings.
FDA emphasized the importance of a more reliable follow-up PCR test, especially to symptomatic individuals who tested positive for COVID-19 infection or those who have been exposed to the novel coronavirus.
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