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. Picture: Bongani Mbatha /African News Agency (ANA)
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. Picture: Bongani Mbatha /African News Agency (ANA)

3 reasons why a negative Covid-19 test doesn't necessarily mean you're not infected

By The Washington Post Time of article published Jan 14, 2021

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By Marisa Lati

Washington - Rampant community transmission of the coronavirus and a crippled vaccine rollout have boxed people into a corner: Many are relying on negative test results, inherently limited in usefulness, as a certificate of clearance to see family and friends.

While testing is critical to containing the outbreak, a negative test result is not a free pass to forgo social distancing, mask-wearing and large gatherings.

Here are a few reminders about what a negative coronavirus test means and doesn't mean.

The tests still have varying degrees of accuracy

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.

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%, but falls to about 38% 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%, 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.

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.

The Washington Post's Natalie B. Compton, Melody Schreiber and William Wan contributed to this report.

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