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Days after President Trump tested positive, White House press secretary Kayleigh McEnany has also tested positive for COVID-19.

USA TODAY

White House Press Secretary Kayleigh McEnany said she tested negative for COVID-19 “every day since Thursday” – until she tested positive Monday. 

McEnany’s disclosure shows the limits of daily testing in halting the spread of the coronavirus that causes COVID-19. A person might have such small traces of the virus after exposure a case isn’t identified until days later, even using the most accurate and sensitive tests.

It’s “what we call a window period: that’s the period after you get infected and before you test positive,” said Dr. Richard Scanlan, chair of the College of American Pathologists Council on Accreditation.

The widening outbreak of people connected to President Donald Trump brings new scrutiny to the White House’s strategy, which relies on testing everyone who comes in close contact with the president. But experts say a coronavirus test is a just snapshot at any given moment.

Some public health experts criticize the White House for putting so much weight on frequent testing. Test specificity ranges across the board – some need only small amounts of virus particles to produce a positive result, while others require tens of thousands of particles.

people positive around trump (Photo: Javier Zarracina)

The most sensitive type of test is polymerase chain reaction, or PCR, which amplifies a small amount of DNA sample from a nasal swab. PCR tests are usually conducted at labs and can take days to process, but some portable machines can deliver results in less than one hour.

PCR tests can usually detect a virus four days after a person is infected, Scanlan said. 

Antigen tests, which detect proteins of the coronavirus, are less expensive and deliver results more quickly than most types of PCR tests.

The U.S. Department of Health and Human Services has purchased thousands of antigen testing machines and sent them to nursing homes. HHS also purchased 150 million rapid antigen strip tests called BinaxNOW test, made by Abbott Laboratories. 

But antigen tests are less sensitive than PCR tests and often don’t detect an infection until about seven days after a person has been infected, Scanlan said.

A person who becomes sick typically develops symptoms four or five day after exposure. But symptoms can can range from two to 14 days after infection, and many people never get sick. 

“So the PCR test starts picking up the virus about a day before you develop symptoms, whereas the antigen test might be a couple of days after you develop symptoms,” Scanlan said.

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