What Coronavirus Test Results Do — And Don't — Mean : Short Wave Even though we've been living with the pandemic for months, there's still lots of confusion about coronavirus tests and what the results do — and don't — mean. NPR correspondent Rob Stein explains the types of tests, when they are most accurate and how to make sense of the results.

Email the show at shortwave@npr.org.
NPR logo

What Coronavirus Test Results Do — And Don't — Mean

  • Download
  • <iframe src="https://www.npr.org/player/embed/920924567/921317107" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
What Coronavirus Test Results Do — And Don't — Mean

What Coronavirus Test Results Do — And Don't — Mean

  • Download
  • <iframe src="https://www.npr.org/player/embed/920924567/921317107" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

MADDIE SOFIA, HOST:

You're listening to SHORT WAVE from NPR.

Hey, everybody. Today we've got NPR correspondent Rob Stein on the line to talk all about coronavirus testing. Hey, Rob.

ROB STEIN, BYLINE: Hey, Maddie. Long time no see.

SOFIA: Too long, Rob. We've missed you over here on SHORT WAVE.

STEIN: Oh, man. I missed you guys so much. It's so nice to be back.

SOFIA: Yes (laughter). All right, Rob. So even though we've been living through the pandemic for a while now, there's still lots of confusion about coronavirus tests and what the results of those tests do and don't mean.

STEIN: For sure, Maddie. And that's become even more evident as lots of people in the Trump administration, including President Trump himself, have tested positive in the last week.

(SOUNDBITE OF MONTAGE)

UNIDENTIFIED REPORTER #1: Trump administration is fighting to contain an outbreak of coronavirus.

UNIDENTIFIED REPORTER #2: In 48 hours, two members of the White House staff tested positive for the virus.

UNIDENTIFIED REPORTER #3: The White House has been relying heavily on testing to stop the spread, but it's far from foolproof.

UNIDENTIFIED REPORTER #4: The vice president's press secretary, Katie Miller, tested positive.

SOFIA: Testing is important to be sure, but the administration has consistently over relied on testing to protect people working in the White House instead of fully committing to all the necessary precautions like consistent mask wearing and social distancing and being outside - that kind of stuff.

STEIN: Exactly, Maddie. Tests are really important. But what we're seeing unfold right now also shows their limitations, that tests aren't some kind of, you know, magical force field. So it's important to know what kind of test you're getting and their strengths and their weaknesses.

SOFIA: So today on the show, we go back to the basics of coronavirus testing, the types of tests, when they're the most accurate and what their results do and don't mean.

I'm Maddie Sofia, and you're listening to SHORT WAVE from NPR.

(SOUNDBITE OF MUSIC)

SOFIA: So, Rob, we're talking about testing today. Let's go through the two main types of tests to see if somebody is infected with the virus - antigen tests and molecular tests, the most common molecular test being PCR tests.

STEIN: Right. PCR looks for genetic material from the virus. PCR tests are really good, but they're also pretty complicated. They have to be done in labs with specialized equipment, trained technicians, and that's a big reason why they've been so hard to get.

SOFIA: OK, so that's PCR. What about antigen tests, Rob?

STEIN: Yeah. So antigen tests look for proteins from the virus instead of the genetic material, and they're just starting to become more widely available. The big advantage of antigen tests is that they're a lot cheaper and easier to make into use. They can produce results really fast - in minutes instead of hours or days - right on the spot. But there are lots of questions about antigen tests. One big one is a concern that they're less reliable. So sometimes they're used as kind of an initial test, which is then confirmed later by a PCR test.

SOFIA: Got it. Got it. So PCR is the more accurate test, but it takes longer.

STEIN: Right.

SOFIA: But one thing both of these tests have in common is that the timing of when you take the test is important. After being exposed, you don't just test positive immediately. It takes a while for the virus to replicate and to make enough of itself that it's actually detectable.

STEIN: Exactly. And we have to say here that we don't have a ton of data on this, and the data we do have isn't completely cut and dry.

SOFIA: Yeah, that's true, Rob. Caveat appreciated as always, Rob (laughter).

STEIN: Oh, yeah. So, you know, Maddie, that said, let's focus on the PCR test. Let's say you're infected on, oh, let's say, a Monday. We'll call that Day 1. On Day 2, the day after exposure, there is a tremendously high probability of you testing negative, meaning the test says you're negative for the virus even though you're really infected. But over the next three or four days, the probability of false negative drops. But it still can happen.

SOFIA: Yeah, I mean, in one study that I was looking at, Rob, the tests were most accurate about eight days after you'd been exposed.

STEIN: Yeah, yeah, yeah. And this is one reason that people can test negative for days before finally testing positive. You know, for example, could be why White House Press Secretary Kayleigh McEnany tested negative for days and then, boom, positive.

SOFIA: Yeah, exactly. And this kind of tricky lag period is why - and everybody listen up out there - if you are ever exposed to someone who you know has coronavirus, you need to quarantine yourself for 14 days to make sure you aren't sick and you're not getting other people sick.

STEIN: Yeah, testing negative for a few days after exposure is definitely not enough to know you haven't been infected. In fact, if there's just one thing anyone takes away from this discussion today, it's this - no one can ever absolutely rely on one negative test as 100% guarantee that they're not infected, that you're totally safe to say, you know, go visit grandma or enter somebody else's bubble or go to work.

SOFIA: Right.

STEIN: That's because these tests, they can misinfect (ph) people for lots of reasons, not just because they get tested too soon, but, you know, maybe the sample was collected wrong. Or maybe the sample got messed up somehow on the way to the lab. Or maybe the analysis was done incorrectly. There are just lots of reasons tests can miss people. And it's important to remember, just because you test negative today doesn't mean you won't catch the virus right after you get tested.

SOFIA: Right, Rob. And, I mean, with this virus, it can take a while for symptoms to show up, if they show up at all. So you can't really bank on symptoms showing up to let you know that you're sick. Like, for some context here with the flu, in cases where people do get symptoms - because they don't always get symptoms - flu symptoms normally show up one to four days after you've been exposed. So with the flu, you know you're sick pretty quickly. And that's typically not the case with coronavirus.

STEIN: Yeah, you got it. With this coronavirus, a person typically doesn't develop symptoms for five days after exposure - and sometimes even longer, sometimes a lot longer. So with the coronavirus, you can walk around infected for a really long time without knowing it.

SOFIA: Yeah. And like we said, that's if you do develop symptoms. We've learned that some people never develop symptoms, but they can still get other people sick. And we call those asymptomatic cases. So, I mean, Rob, how can people know if they're safe and they won't spread the virus to somebody else or, you know, someone they know won't spread it to them?

STEIN: Yeah, so that's the question everybody wants to know the answer to. And the only way to really know is to be really, really careful for at least 14 days. Make absolutely sure you don't come into contact with someone else who could possibly have gotten infected and spread the virus to you. And then, OK, sure, get tested for a bit more peace of mind that you did everything right. But it's not the test that really tells you that. It's your behavior. It's what you do...

SOFIA: Yes.

STEIN: ...Wearing those masks, stay at least 6 feet away from other people, trying to get together with other people outdoors or in well-ventilated areas, washing your hands all the time. All that advice that we've been hearing about over and over and over again for so long, you know, those measures, they really do work.

SOFIA: Preach it, Rob. Preach it. I'm here for all of that. I'm here for all of that.

STEIN: It's about time. When are you going to get the message, Maddie?

SOFIA: Rob, I've been in this closet for six weeks. That's the only place I go. You know that (laughter).

STEIN: Yeah, but who's in there with you?

SOFIA: (Laughter) Oh, my God. All right, Rob. OK. So we just talked a lot about the limitations of testing. But I think we should be clear with the audience here. Testing is really important, right?

STEIN: Yeah. Testing has its limits, but it's essential. You know, that's how we spot people who catch the virus as soon as possible to prevent them from spreading to other people and to track down and test anyone else who might have gotten infected by them. That's how we stop outbreaks before they start.

SOFIA: Right.

STEIN: And nationally, you know, we are slowly seeing more testing. And we're about to see way, way more testing and more types of testing. In fact, there's been a lot of talk lately about how these antigen tests could be a game-changer because they're so cheap and so easy to make and use, tens of millions of them could become available.

SOFIA: Right.

STEIN: And they produce results right away, right on the spot. So eventually, if we do get enough of them, they could provide the kind of widespread screening, testing that we need to regularly test students and teachers and waiters and bartenders and other workers. And that's what we really would need to do to allow the country to finally reopen safely.

SOFIA: Yeah. And, I mean, we should say here, Rob, that antigen tests do tend to produce more false negatives than those genetic tests, right?

STEIN: Yep. Absolutely.

SOFIA: And there's also a big concern about false positives.

STEIN: Yeah.

SOFIA: And, you know, we haven't talked about it a lot, but false positives can cause their own sets of problems, right?

STEIN: Big problems sometimes, really big problems.

SOFIA: Yeah. But if we figure out how to use them the right way, they could make a big difference.

STEIN: Right. Absolutely. And that's the hope that we could eventually get there. And that could be a real paradigm shift in the way testing is done in this country.

SOFIA: Yeah.

STEIN: But, you know, big picture, no one ever thought we could test our way out of this pandemic. It's part of the strategy - a key part but, really, just part. One researcher I was talking to about this compared testing to, say, you know, airbags in cars. They can save a lot of lives, but you still need seatbelts and reinforced doors in the same way we still need testing and masks and staying 6 feet away from other people and spending as much time outside as possible. You need to do all of that to stay safe and maybe, hopefully, finally get the virus under control, so we can start to get some version of our old lives back.

SOFIA: All right. Rob Stein, NPR correspondent - so good to have you back, bud. We appreciate you.

STEIN: Oh, it's so nice to be back. Thanks, Maddie. I appreciate you.

(SOUNDBITE OF MUSIC)

SOFIA: This show was produced by Brit Hanson, fact-checked by Ariela Zebede and edited by Gisele Grayson and Viet Le. I'm Maddie Sofia. Thanks for listening to SHORT WAVE from NPR.

Copyright © 2020 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.