COVID-19 Q&A: Which is better, the deep nasal swab or the rapid test?

LANSING – When it comes to detecting the coronavirus, not all tests are created equally.

Two tests are widely available to see if someone has the virus: a polymerase chain reaction swab, or PCR test, and a less invasive antigen test. While both methods have the same goal, Jon Baker, Sparrow Health System’s administrative director of laboratory services, said the rapid antigen tests fall short of the PCR test.

“The main difference is that there’s a much greater possibility of a false negative with an antigen test,” Baker said. “Particularly if you don’t have symptoms.”

The PCR tests, in which a sample is collected with a deep nasal swab, have a higher sensitivity to detect the coronavirus. Many rapid tests, which involve either a shallow nasal swab or throat swab, don’t have the same capability, Baker said.

“PCR is by a long shot the most sensitive method,” he said.

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But that doesn’t mean the rapid tests don’t have their uses, Baker said.

Why is the PCR test better?

The PCR test has become the “gold standard” because it is sensitive enough to detect the smallest traces of the virus, Baker said.

If you’ve been exposed to the virus, were in contact with someone who tested positive or are only showing mild symptoms, the PCR test is the best method, Baker said.

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The limitations of the PCR test are that it takes longer to process and not all medical facilities can perform the tests, Baker said. The antigen tests can fill that gap but it should be used in concert with a confirmatory PCR test if one is available, he said.

What are rapid tests good for?

The antigen tests are best for people who already show symptoms of the virus and are within a five- to 12-day window since symptoms started, Baker said.

While the antigen tests are less sensitive, they will most likely generate a positive result for someone with a large viral load, Baker said. If people are asymptomatic or only in the early stages of contracting the virus, an antigen test might not catch its presence, he said.

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The antigen tests are a useful tool to regularly test in congregate settings, like nursing homes and jails, Baker said. If people need to get tested regularly, the rapid test is the best method because of the time required to process a PCR sample.

Contact reporter Craig Lyons at 517-377-1047 or calyons@lsj.com. Follow him on Twitter @craigalyons.