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Getting Answers: The science behind people testing negative then positive for coronavirus

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COVID-19 swab test. File photo

MADISON (WKOW) -- Medical experts say for people who've been exposed to COVID-19, a single negative test isn't a guarantee they're virus-free.

"The bottom line is a single test that's negative in someone who is at risk for COVID because they've been exposed doesn't mean much," Dr. Nasia Safdar said Wednesday. Safdar is the Medical Director of Infection Control for UW Health.

Safdar said there are several reasons someone might test negative even if they have the virus.

Often, this will happen if someone gets tested too soon to the time they were exposed.

"After exposure, we have to build up enough virus in our body in order for the test to detect that," Dr. Amy Franta, the regional chief medical officer for SSM Health, said.

Franta said that's why many doctors recommend people wait five to seven days after they were exposed to get tested. She said if someone is symptomatic, they don't need to wait as long because they most likely have enough virus built up for the tests to detect.

Another reason someone could test negative and then test positive comes from the types of tests used.

In the case of UW Badgers football coach Paul Chryst, his negative test result came from an antigen test, which provides quick results.

"It's a tradeoff," Franta said. "The test might give us a quicker result, but it may not be as accurate."

His positive result came from a PCR test, which Safdar said is the gold standard. The community testing site at the Alliant Energy Center uses PCR tests.

Even though PCR tests are more accurate, they don't provide complete certainty with negative results.

For someone who has been exposed to coronavirus, it could take up to 14 days for the virus to accumulate to detectable levels.

"It's entirely conceivable that one day you'll be negative but because you're still in that 14 day period, you could be positive the following day," Safdar said.

That appears to be what's happening with the Badger football team, which has had 6 players and 6 staff members test positive since their game against Illinois last Friday.

Safdar and Franta said because people can be infectious without knowing they're sick, it's important for anyone exposed to quarantine for 14 days, regardless of if they test negative.

"For people who are exposed but are not showing symptoms, a negative test is really just a snapshot in time," Safdar said. "[It] is not necessarily permission to come out of that quarantine...You still have a few days in which you could develop symptoms."

Safdar said if you have tested positive, you don't need to get tested again to prove you've recovered.

The PCR test identifies virus genes, which Safdar said can linger for several weeks after you have recovered and are no longer infectious.

She said though some workplaces or groups are requiring people who have COVID-19 to test negative before they return, she disagrees with the practice.

"To me, that's a flawed way of looking at it because the PCR test can stay positive for many weeks after the initial episode," she said.

She said it's more effective to require anyone with COVID-19 to isolate for 10 days and be symptom-free before returning to normal activities.