UW doctor: When to use a PCR versus rapid antigen test for COVID-19

As we close move into the second fall season of the COVID-19 pandemic, the demand for rapid tests is still growing, but not all tests are exactly the same.

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Dr. Geoff Baird with UW Medicine has a simple formula for weighing the two main test types: PCR (polymerase chain reaction) and rapid antigen.

“If someone is symptomatic, it makes the most sense to take the test that is actually available at that time,” Baird said. “The clinical performance of a rapid antigen test is good enough such that you will get a pretty good result, a pretty reliable result, in that context.”

However, he says antigen-based rapid tests aren’t as sensitive as PCR tests at detecting COVID. That’s  an important difference when testing asymptomatic people who often experience more false positive or false negative results with rapid antigen tests.

“If you have a swab and there’s about 100 or 200 viruses, we can detect that with a PCR. For a rapid test, an antigen test, there usually needs to be tens of thousands or maybe hundreds of thousands, and so there’s just a lot more virus that needs to be there,” Baird said.

For that reason, he says PCR testing is a more reliable choice for people who don’t feel sick but who want a test result to travel, attend an event, or gather with family.

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Dr. Baird is the chair of laboratory medicine and pathology at UW Medicine.

The KIRO Radio Newsdesk contributed to this report.

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