Exposed to COVID-19 and got negative test result? Here's why you should isolate anyway
People who test negative for COVID-19 after they were exposed to someone with the virus should still take precautions, according to North State public health officials.
That’s because there is a slight chance those who test negative may still have the virus, said Dr. Karen Ramstrom at the Shasta County Health and Human Services Agency (HHSA).
With the kind of test kits widely used throughout Siskiyou and Shasta counties — RT-PCR test kits, “the likelihood of false positives is very, very low,” Ramstrom said at a public health briefing on Wednesday. “We (doctors) worry more about false negatives."
A false negative result on a COVID-19 test is a result that says a person does not have COVID-19 when they do.
While they are unlikely, when they do occur with a test, it's usually for one of two reasons, Ramstrom said in an interview with the Record Searchlight:
- The timing of the test is off, so a person has low levels of virus in their system.
- The sample collected from the nose or throat of a person does not include enough cells to adequately test for the virus.
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If a false negative test results from problems with the specimen collection — the swabbing of the of cells collected from the person's nose and throat, it means there isn't enough evidence of the virus in the specimen swabbed from the person.
Ramstrom explained the timing of the test can cause false negative results if the person had COVID-19 for a while and the body does not have enough of the virus in the throat or nasal passages for the test to detect it.
According to information published by Harvard Medical School, false negative results are also more likely if a person is tested too soon: Like on the day they were infected, or close to it. That's "because there are not yet enough viral particles in your nose or saliva to detect." Chances of a false negative test result go down if someone is tested a few days after they were infected, or after they develop symptoms.
The possibility of a false negative test result in a person who has the virus is why we recommend those who test negative, but were exposed to COVID-19 or have some symptoms, still isolate themselves, Ramstrom said.
How PCR tests work
Molecular tests, like the RT-PCR test used throughout the North State, work by detecting the virus’ genetic material, according to the FDA.
RT-PCR tests are administered by swabbing a person’s throat and inner nose. The tests look for evidence of the coronavirus’ RNA — it's genetic material — on the swabbed throat and nasal samples.
“People are more likely to have those particles of viral RNA in their nasal passages or their throat passages earlier in infection,” Ramstrom said.
Taking a different kind of test, like an antigen test, or retaking the test later can provide more information if a person tests negative, but there are suspicions they might still have the virus.
While negative tests harbor possible errors, the same is hardly ever true of a positive test, according to Harvard. "Generally speaking, if a test result comes back positive, it is almost certain that the person is infected."
- Read more about testing in Siskiyou County, and where to go to get a test at www.co.siskiyou.ca.us/publichealth/page/covid-19-testing.
- Read more about testing in Shasta County, and where to go to get a test at www.co.shasta.ca.us/covid-19/get-tested.
- Read more from the FDA about reverse transcriptase–polymerase chain reaction tests (RT-PCR tests) at https://bit.ly/3bkjGZP.
Jessica Skropanic is features reporter for the Record Searchlight/USA Today Network.