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Causes of a false negative covid test

The tests still have varying degrees of accuracy

Coronavirus testing in the United States has increased 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) test – considered the “gold standard” – and less accurate antigen tests.

PCR testing uses a molecular process and is processed in a laboratory. Faster and cheaper antigen testing detects proteins on the surface of the virus. Most quick home tests that are estimated to be approx. 90 percent accurate, using antigen technology.

PCR testing is your best choice to get a sense of whether you are infected, although these results take longer than antigen results. PCR testing can still result in false negatives.

This is partly because the giant cotton swabs used in many of these tests may not collect all traces of the virus deep inside the nasal cavity. The virus may have moved from your nose to your lungs or somewhere else, or the cotton swab may not have been inserted far enough.

But nasopharyngeal cotton swabs that collect samples from the back of your nose and throat are still more reliable than less invasive swab sticks and nose sticks.

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

Early in an infection, the virus may not have reproduced enough to be detected. The false negative rate of PCR testing on the day of exposure is 1

00 percent, but drops to approx. 38 percent five days later, when symptoms usually occur, according to an analysis published in the Annals of Internal Medicine. The rate drops further to about 20 percent after another three days.

“We do not yet understand exactly when an infected person begins to test positive for the virus,” said Muge Cevik, a clinical associate professor of infectious diseases and medical virology at the University of St. Louis. Andrews, to the Washington Post in December. “So there are situations where a person can test negative, but they can still be contagious.”

If you think you have been exposed to coronavirus but are receiving a negative test result, you may be tested again in a few days. You should also quarantine yourself while waiting for results.

The Centers for Disease Control and Prevention proposes quarantine for 14 days, but considers 10 days acceptable if you have not experienced symptoms. The agency also says it is likely safe to leave your home after seven days if you have no symptoms and have received a negative test result.

You can test negative and get infected shortly after

In addition to the other limitations, a coronavirus test only captures a moment in time. You can walk out of a test center and get the virus immediately. The test you just took does not reflect your new infection.

This limitation is especially important to keep in mind if you plan to travel once you have learned the result of your test. An overcrowded airport terminal or rest stop toilet can expose you to the virus once you have concluded that a negative test means you are not infected.

Gathering indoors is inherently risky, and a negative test result does not tell the full story.

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

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