Rapid COVID-19 tests: Best use depends on the situation

As the COVID-19 pandemic persists into a second fall season, rapid diagnostic tests have proliferated. Dr. Geoff Baird, chair of laboratory medicine and pathology at UW Medicine, has a simple formula for weighing the two main test types: PCR (polymerase chain reaction) and rapid antigen.

Do you have COVID-19 symptoms?

“If someone is symptomatic, it makes the most sense to take the test that is actually available at that time,” Baird says. “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.”

Baird says rapid antigen tests found at drugstores are useful to stash in your medicine cabinet, so you can have them on hand if you start having symptoms. Antigen-based tests can yield results in a matter of minutes but are less precise than PCR tests, which trace even the smallest of viral load. 

This difference is important when testing asymptomatic people, who more often experience false positive and false negative results with rapid antigen tests. Baird says PCR testing is the 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.

“You really want to be sure that that person doesn't even have 100 viruses in their nose, and the way that you can detect that is with PCR.”