Here's how coronavirus lab tests really work, and why they don't always give satisfying results
- 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.
- This step-by-step guide explains how coronavirus swab-the-nose-and-throat tests work, and why the laboratory-based ones often require more than a day to produce results.
- Don't assume that just because your test is negative, you are safe. Even if you have the coronavirus, the test may not turn up positive.
- Visit Business Insider's homepage for more stories.
Do I have the coronavirus?
That seemingly simple question is tricky to answer definitively.
For one, the tell-tale symptoms of Covid-19 (the disease caused by the virus) can often mimic other illnesses, such as the flu or a common cold.
Getting tested for Covid is also not a surefire way to know one way or the other if you have the coronavirus.
The most precise type of testing for Covid-19, the molecular kind, 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 sick 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.
Still, these types of laboratory-confirmed tests (not to be confused with the less accurate, rapid, 15 minute kinds of nasal swabs that are becoming increasingly available nationwide) work fairly well.
Generally speaking, molecular coronavirus tests catch more than seven out of every 10 Covid-19 infections they hunt down.
Here's how these real-time RT-PCR*tests actually work, 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 estimated that roughly 30% of coronavirus tests may come back negative, even when a person actually has the virus in their body. Another scientific review of coronavirus PCR tests, published in May, suggested similarly that false negative results occur between 2% and 29% of the time. (The chances of a false positive test, which would alert a patient that they have the virus when in fact they don't, are much slimmer.)
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 if you've had Covid-19 in the past; the test is only designed detect an ongoing infection. (Blood tests that search for the presence of coronavirus antibodies do that.)
*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.
This story was originally published on April 15, 2020. It has been updated with new information.
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