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Reasons for a false negative covid test

The tests still have varying degrees of accuracy

Coronavirus testing in the United States has ramped up significantly in recent months, with the daily test count hovering around 1.5 million in January. There are two categories of tests: PCR (polymerase chain reaction) tests — considered the “gold standard” — and less-accurate antigen tests.

PCR tests use a molecular process and are processed in a lab. Faster and cheaper antigen tests detect proteins on the virus’s surface. Most rapid home tests, estimated to be about 90 percent accurate, use the antigen technology.

PCR tests are your best bet for getting a sense of whether you’re infected, even though those results take longer than antigen results. Still, PCR tests can result in false negatives.

That’s in part because the giant cotton swabs used in many of those tests may not pick up all traces of the virus deep in your nasal cavity. The virus may have moved from your nose to your lungs or somewhere else, or the swab may not have been inserted far enough.

But nasopharyngeal swabs, which collect samples from the back of your nose and throat, are still more reliable than less invasive throat swabs and nasal swabs.

A test may not be able to detect the virus early in its course

Early in an infection, the virus may not have reproduced enough to be detectable. The false negative rate of PCR tests on the day of exposure is 100 percent, but falls to about 38 percent five days later as symptoms usually set in, according to an analysis published in the Annals of Internal Medicine. The rate decreases further, to about 20 percent, after three more days.

“We don’t yet understand exactly when a person who’s infected will start testing positive for the virus,” Muge Cevik, a clinical lecturer in infectious diseases and medical virology at the University of St. Andrews, told The Washington Post in December. “So there are situations when a person could test negative, but they could still be contagious.”

If you think you’ve been exposed to the coronavirus but receive a negative test result, you may want to get tested again in a few days. You should also self-quarantine while you’re waiting for results.

The Centers for Disease Control and Prevention suggests quarantining for 14 days but considers 10 days acceptable if you haven’t experienced symptoms. The agency also says it’s probably safe to leave your home after seven days if you have no symptoms and have received a negative test result.

You could test negative and become infected soon afterward

On top of the other limitations, a coronavirus test captures just one moment in time. You could walk out of a testing center and immediately contract the virus. The test you just took is not going to reflect your new infection.

This constraint is particularly important to keep in mind if you plan to travel after you learn the outcome of your test. A crowded airport terminal or rest-stop restroom could expose you to the virus after you’ve concluded that a negative test means you’re not infected.

Gathering indoors is inherently risky, and a negative test result doesn’t tell the full story.

Natalie B. Compton, Melody Schreiber and William Wan contributed to this report.

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