Here’s how coronavirus nose and throat swab tests really work, and why they don’t always give satisfying results

Samantha Lee/Business Insider
  • One of the best ways to figure out how far a virus like COVID-19 has spread in a community is to test for it.
  • In the US, massive federal delays rolling out COVID-19 testing have led to a situation where it’s hard to track where new cases may be spreading in real time.
  • Here’s how a coronavirus swab-the-nose-and-throat test works, why it takes so long to produce results, and why it doesn’t always turn up positive, even if a person has (or had) COVID-19.
  • Visit Business Insider’s homepage for more stories.

Do I have the coronavirus?

That seemingly simple question is proving exceedingly hard to answer definitively. In many places around the US, it’s difficult to even get tested for COVID-19, the disease caused by the coronavirus.

With the virus now spreading across all 50 states, there are far more people in the US who want coronavirus tests than can get them, and those who get tested do wait days for their results. More than 1.2 million people nationwide have gotten a COVID-19 test, and about 17% of those test results have come back positive, according to the US Centres for Disease Control and Prevention.

For those who do manage to get tested, the test itself is also painful.

A test for COVID-19 starts by reaching deep inside a person’s nose and throat to extract sputum – the gunk that gets ejected through coughing, sneezing, spitting, and even singing. That throat gunk, in turn, can be tested for the presence of some of the coronavirus’ tell-tale genes.

But, if a person’s infection isn’t living in the spot where they are swabbed, their infection isn’t caught at the right moment, or their sample isn’t collected properly, their test could still come back negative.

Here’s how the most common kind of coronavirus test, real-time RT-PCR*, actually works, from start to finish:

“Who you test, and when they test, and when you test them, and the specimen that you use to collect all really impact the performance of the test,” Dr. Jana Broadhurst, director of the Nebraska Biocontainment Unit Clinical Laboratory, recently told reporters on a SciLine conference call.

Broadhurst estimates that roughly 30% of coronavirus tests may come back negative, even when a person actually has the virus in their body.

Coronavirus testJim Urquhart/ReutersHealth care worker tests people at a drive-thru testing station run by the state health department, for people who suspect they have novel coronavirus, in Denver, Colorado, March 11, 2020.

That’s why if you have COVID-19 symptoms, it’s best to stay home for 14 days and self-quarantine, no matter what your test results may show.

Real-time RT-PCR tests can’t tell you if you’ve had COVID-19 in the past; the test is only designed detect an ongoing infection. Scientists are racing to create new kinds of blood tests, which will help determine who’s had COVID-19 and who might be immune to it, even if they never had coronavirus symptoms before.

(*Note: RT-PCR stands for reverse transcription polymerase chain reaction. Reverse transcription turns viral RNA into DNA that polymerase enzymes can then properly amplify. The process is also called real-time RT-PCR, because the PCR reaction is measuring the amplification of coronavirus genes in real time.)

Special thanks to Dr. Jeffrey SoRelle, Dr. Alex Greninger, and Business Insider’s quantitative editor Andy Kiersz for their expert guidance on the inner workings of real-time RT-PCR.

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