What goes on inside those test tubes to detect whether a person is positive for SARS-CoV-2, the virus that causes COVID-19?
How Coronavirus Tests Work
Published: March 27, 2020
Nurse: All right, all done.
Yeah, it’s awful. I’m sorry. I wish there was a better way to do it. Are you OK?
Shelby Anderson: Yeah.
It was quite painful, not gonna lie. You know like when you get a shot and it kinda like, pinches a little bit? It was like that in your nose.
Sarah Epps: The doctor comes to your car and asks you a bunch of questions, does a physical exam with a stethoscope and everything, and then does the swab and sends you on your way and that’s it.
It felt like you were snorting really spicy mustard up one nostril, it was quite a shock.
Narrator: This is how most coronavirus tests are done in the U.S.
John Connor: So you stick a swab into somebody’s nose and you pull it out. And what's on it? You have some of your cells. You have potentially, virus.
Narrator: To find out if the virus is on that swab, scientists use a process known as polymerase chain reaction, PCR.
David Price: It’s basically just a way of taking a small amount of nucleic acids, RNA specifically for this virus and turning it into a lot of nucleic acid so that you can detect it
Narrator: The new coronavirus contains RNA—a single strand of genetic information that sits inside a spherical casing.
Connor: So the first thing you have to do is to get into that genome. So after this swab, what you will then do is take what was on that swab and blow everything apart.
Narrator: Chemicals are added to isolate any genetic material.
Pride: And then you take that tube and you subject it to a PCR.
Narrator: Pieces of genetic material—called primers—are added to the tube.
Connor: So, the primers are those small pieces of DNA that are going and looking for the genomic RNA. And if the genomic RNA is there, those primers will bind and that binding makes a landing site for the enzyme to make that new copy.
Narrator: This happens over and over again—creating millions of copies.
Connor: As that new copy is made, now you get fluorescent molecules coming in and binding to that new copy.
Narrator: So if that snippet of coronavirus genetic material is present, PCR will make lots of copies, and fluorescent molecules will make them glow. That glow means patient is infected.
In February, the CDC sent out test kits for this PCR process, but some of the chemicals didn’t work properly.
Pride: What happens is that for every test you run a negative control, which is a sample that's supposed to be negative that also tells you if the test is working. If your negative control is positive, the whole thing you've got to throw in the trash, basically.
So, what was happening with the CDC tests is that there was a lot of false positives particularly in the negative control and that was due to the impurities in the kits.
Narrator: In March, the FDA loosened regulations, allowing labs and commercial testing facilities to use their own tests.
Pride: So that's that's really sort of a significant step and that's what allowed us to go live with the test.
Narrator: Now there are more challenges.
Pride: Even just getting materials to extract the RNA from the virus is a problem right now. Every step of the way, we are experiencing bottlenecks.
Narrator: Shortages of protective gear limit health care workers’ ability to safely take samples. And shortages of test kits and crucial chemicals limit labs’ ability to test the samples for the virus.
Pride: So every day when I come into work, the first thing I do is start getting on the phone, getting on my emails, trying to secure additional supplies the kits themselves, all of the testing materials. We're basically using all our materials much, much faster than we would like to.
Narrator: Most PCR tests are run on machines at specialized labs that process many samples at once. Some labs are able to share results quickly.
Anderson: I had mine in 18 hours.
Narrator: Shelby says her test came back negative.
Anderson: No, I don’t have it.
Narrator: Some results have taken several days.
Epps: I’m still waiting for my test results.
Narrator: Sarah reports her results came back an entire week after her test, and were also negative.
Some new tests can be done much faster—in less than an hour—but these will likely be used primarily for patients in hospitals and some health care workers.
The FDA is warning against new at-home coronavirus tests, calling them “fraudulent.”
A completely different kind of test is being developed to detect the virus by looking for antibodies.
Connor: Our immune system goes into a very nice adaptive response where it will identify the infecting virus and make antibodies that have very high affinity against it. And those are effective at blocking further infection.
Narrator: An antibody test could not only identify who’s been infected, but also identify who may have immunity.
Connor: Those could be people that are very important in our response to the disease. If they already have an immune system that has seen the virus and has developed an effective response against it, they might be people that can help individuals that do need assistance when they are sick.
Narrator: Scientists are optimistic about improving capacity to test for the virus using methods currently in use.
Connor: I am optimistic about the coming weeks. I think we will get to the point where we understand this virus. We have the diagnostic capacity.
Pride: As multiple different companies are approved and they ramp up their production, we should eventually be able to meet the need. The question is, is that two weeks from now or is that six weeks from now? That's the harder part to answer.
Digital Producer: Emily Zendt
Narrated by: Caitlin Saks
Research and Production: Caitlin Saks, Ari Daniel, Robin Kazmier, Sukee Bennett
Additional footage: CDC, Medical Animation provided by Scientific Animations
© 2020 WGBH Educational Foundation