PCR vs. Rapid: Making sense of Covid tests


'A positive test is a positive test. It is COVID.' - Dr Arwady

More people testing for Covid means more questions.

When it comes to testing for COVID19, the FDA say there are two types of diagnostic tests.

PCR Test
Analyzes a sample to see if it contains genetic material from the virus.

Antigen or Rapid Tests
Checks for viral proteins in a sample taken from inside your nose.

Matt Charles, Assistant Vice President of Laboratory Services for NorthShore University Health System said PCR tests are usually more sensitive at picking up on positive Covid samples.

PCR is considered the gold standard for Covid testing and the antigen tests are measured against that when they’re evaluated,” he said. “So typically, you expect to see about 70-80% sensitivity as compared with a PCR-based test.”

However, there are other variables that can come into play.

Dr. Allison Arwady, Chicago’s top doctor, said when you take a test matters. And regardless of the type of test, if it’s positive, trust that you have Covid.

Neither of these tests are entirely perfect, but they’re both very, very good,” she said. “It mainly depends when you take the test in your infection what it’s going to show. Bottom line, right now, a positive test is a positive test. It is COVID.”

The FDA has identified some PCR tests that failed to detect the new Omicron variant. Those tests are listed on the FDA’s website.

“Any of the laboratory systems out there are currently, (and) always watching those studies from the NIH and from the FDA to make sure that the tests that we offer are effective,” Charles said.

When it comes to rapid tests, early data into the FDA suggests some antigen tests may have reduced sensitivity in detecting the omicron variant.

That’s something the FDA and test developers continue to evaluate.

When used properly, the at-home antigen tests catch about 85% of COVID cases, reports the New York Times. They’re better at detecting COVID cases when someone is symptomatic (and therefore has a higher viral load) than someone who is asymptomatic and has a lower viral load.

The sensitivity to COVID-19 presence won’t be as high as a PCR test sent to a lab, but that may be OK for your purposes. Dr. Bob Wachter, chair of the Department of Medicine at the University of California, San Francisco, says he’s a fan of the rapid tests you can find at your local pharmacy, such as BinaxNOW.

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