https://www.washingtonpost.com/health/2021/01/13/false-negative-covid-test/
Three reasons a negative coronavirus test doesn’t necessarily mean you’re not
infected
Marisa Iati
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 other mitigation
measures. Here are a few reminders about what a negative coronavirus test means
and doesn’t mean.
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.
8 facts about the coronavirus to combat common misinformation
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.”
The covid-19 symptoms to watch out for
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.
Catch up on the most important developments in the pandemic with our
coronavirus newsletter. All stories in it are free to access.
Natalie B. Compton, Melody Schreiber and William Wan contributed to this report.
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