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


As we approach the second fall season of the COVID-19 pandemic, the demand for rapid tests continues to grow, but not all tests are exactly the same.

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

“If someone is symptomatic, it makes more 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 that you get a fairly good result, a fairly reliable result, in this context.”

However, he says rapid antigen-based tests are not as sensitive as PCR tests for detecting COVID. This is an important difference when testing asymptomatic people who often get more false positive or false negative results with rapid antigen tests.

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

For this reason, he says the PCR test is a more reliable choice for people who don’t feel sick but want a test result when traveling, attending an event, or reuniting with their family.

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

KIRO Radio’s Newsdesk contributed to this report.

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