As COVID and flu collide to create a winter of sick days, it’s not obvious which illness you have. And some people undergoing PCR (polymerase chain reaction) tests for COVID have discovered that their samples are also being tested for influenza and other bugs.
How does this work, and where are COVID tests being used to find more than just COVID?
In brief, PCR tests work by identifying and amplifying viral DNA or RNA.
If you’re looking for the presence of SARS-CoV-2, you add a primer (a specific type of DNA) for SARS-CoV-2 to the sample and it amplifies the presence of SARS-CoV-2 in the sample. (You can learn about PCR tests in more detail in these articles.)
Looking for influenza uses the same trick: you just use an influenza primer rather than one for SARS-CoV-2. The DNA primer added has a different “code”, but all the other parts of the test and equipment used to do it are the same.
You can also use PCR to find other viruses that cause cold symptoms – like rhinoviruses, respiratory syncytial viruses (RSV), or adenoviruses.
All of these tests can be undertaken using the same original swab sample.
If it’s possible to test for all these viruses with PCR, why haven’t we been doing so until recently? Many of us over the past two years have used a PCR test to establish we don’t have COVID, but we definitely have something.
Testing for a variety of things is often done on healthcare workers or people with specific vulnerabilities, but most people don’t benefit from knowing whether they’re sick with a rhinovirus or an adenovirus. Additional tests use additional resources for little advantage.
However, with the unusually early arrival of the flu season, it has become more useful to monitor for influenza. Private providers, in particular, may check for viruses other than SARS-CoV-2, depending on the resources and government rebates available to them.
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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