A negative COVID test, or even several, does not mean you don’t have the virus. Here’s why

A healthcare worker picks up a coronavirus test at a free walk-up COVID-19 testing site at Amazing Things Art Center in Framingham.Jessica Rinaldi/Globe Staff

White House Press Secretary Kayleigh McEnany announced Monday that she has tested positive for the coronavirus, making her the latest member of President Trump’s inner circle to become infected. McEnany noted on Twitter that she had tested negative for COVID-19 every day since Thursday, when the president received his positive diagnosis, and said she is “experiencing no symptoms.”

Her positive result after days of testing negative is a reminder of a central guideline from the Centers for Disease Control and Prevention: A negative coronavirus test doesn’t mean you don’t have the contagion.

“If you test negative, you probably were not infected at the time your sample was collected," the CDC says on its official website. "The test result only means that you did not have COVID-19 at the time of testing. Continue to take steps to protect yourself.”

Note the key word regarding a negative test — probably.

A negative reading, the CDC says, “does not mean you will not get get sick. It is possible that you were very early in your infection when your sample was collected and that you could test positive later.”

How much later? It could take a couple of days, according to Dr. Dani Zander, who chairs the pathology department at the University of Cincinnati College of Medicine.

“A false negative test can happen if one has a very low viral load, which can happen in the first couple of days after infection,” Zander said in a video clip posted to the UC Health website. “Or [it] can happen at the end of the course of the infection. And so, in both of those situations you can get a negative test result, even though you’ve been infected with COVID-19.”

Early in a patient’s infection, she said, “you may have very little viral RNA, and so it may be below the limit of detection of the [testing] instrument. And then that viral load will go up as the infection progresses, and then it’ll come down again as one develops immunity to the infection. And so if you are sampled at different points in that timeline, you may end up with a positive at one point, a negative at another, or vice versa.”

So “where you are in the course of the disease is also important in explaining discrepant results," Zander said.

That’s why the CDC recommends a 14-day quarantine for anyone who has been in close contact with someone who has COVID-19. “... [E]ven if you test negative for COVID-19 or feel healthy, you should stay home (quarantine) since symptoms may appear 2 to 14 days after exposure to the virus,” the government’s own guidelines say.

Though McEnany had been exposed to senior adviser Hope Hicks, who tested positive on Thursday, and attended the Rose Garden ceremony after which several Trump insiders contracted COVID, she chose not to quarantine. As recently as Sunday, the press secretary spoke to reporters without wearing a mask.