The science behind false negative COVID-19 tests

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COVID-19 tests are extremely reliable when they give a positive result, but a negative result can’t always be trusted.

False negatives test results are tests that show a negative result even when the person is infected with the COVID-19 virus, and they are common.

“If you have any COVID-19-like symptoms, you should assume you have COVID-19,” said Melissa Sutton, Oregon Health Authority’s medical director of respiratory viral pathogens.

“Never rely on a negative test result to inform an important decision such as visiting a loved one who might be at risk of severe COVID-19,” said Sutton. “Always assume your stuffy nose, sore throat or other symptoms are a result of COVID-19 and take precautions.”

The basics of testing

The two kinds of COVID-19 tests – rapid antigen tests and polymerase chain reaction (PCR) tests – work to detect the virus in your body, but in different ways.

Rapid antigen tests (the kind used at home that return results in 15 minutes) detect pieces of the virus called antigens. Anything your body’s immune system might respond to, such as viruses, bacteria or toxins, has antigens.

PCR tests, meanwhile, detect the virus’s genetic material.

Because COVID-19 most commonly infects the upper respiratory tract, both tests require a sample from your nose. If enough of the  virus’ genetic material or antigens are present, the tests should detect them.

Science behind false negatives

There are two main reasons the rapid antigen or PCR tests might not work:

  1. The test was done incorrectly.

    “We see clearly in studies that when health care personnel perform a test, it’s more likely to detect the virus than when people test themselves,” Sutton said.

    Some people might be too “timid” when swabbing their nose, she said.  They might not rub against the sides of the nostril, or they might not swab  long enough.  Some testing kits say to circle the swab inside each nostril five times, for example.

    “Everyone who’s been tested at a doctor’s office knows it’s less comfortable than the at-home test,” Sutton said. That’s because the health care provider knows to how to perform the test correctly.

  2. Someone might not be shedding virus particles in their nose.

    Another reason either at-home tests or PCR tests could come back negative despite an active infection is that some  people might “shed the virus differently throughout the course of their infection,” Sutton said.

    The virus can infect cells in many different parts of the body, from your mouth, nose and lungs, to the large and small intestines. Someone could be shedding more virus in their throat or gut than their nose, which would mean a nose swab wouldn’t pick up enough viral particles to show a positive test.

If you have symptoms, take precautions

Because the virus is spreading so widely, if you have symptoms “it no longer matters whether you were exposed to someone who tested positive – take precautions—stay home if you can, wear a mask and avoid individuals at risk of severe disease,” Sutton said.