Experts on Camera

Dr. Susan Butler-Wu: COVID testing this winter

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Diagnostic tests remain one of the most important tools to manage the COVID-19 pandemic.

On Tuesday, December 21, SciLine interviewed: Dr. Susan Butler-Wu, an associate professor of clinical pathology at the University of Southern California, where she directs clinical testing for infectious diseases. See the footage and transcript from the interview below, or select ‘Contents’ on the left to skip to specific questions.

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SUSAN BUTLER-WU: I’m Dr. Susan Butler-Wu. I’m an associate professor of clinical pathology at the Keck School of Medicine of USC here in Los Angeles. And I’m also a clinical microbiologist, and I direct a large, busy clinical microbiology lab at a hospital in LA County. My area of expertise is infectious—testing for infectious diseases. So my day job is overseeing the lab and making sure that we’re doing the right tests that the doctors require, the physicians require. And my research interests have really been focused around that in terms of understanding and comparing and studying, testing for infectious diseases and rapid testing for infectious diseases.

Interview with SciLine

Can testing be used to minimize the risk of spreading COVID-19 during holiday travel and gatherings? If so, what exactly do you recommend people do?


SUSAN BUTLER-WU: I think the important thing to remember with any test is—whether it’s PCR or whether it’s antigen, it’s just reflecting what’s happening up your nose when that swab is taken or when you spit into the tube, if it’s saliva-based PCR test. And so it doesn’t really tell you what’s going to happen in the future. So that’s why it’s important to test as closely to whatever event you’re attending and to understand that it’s not a free pass. There is no test for infectiousness. You absolutely could have infectious virus and test negative. So you have to say this is one step that maybe everyone present’s going to take, but it doesn’t sort of give you a free pass to assume that there’s no way you could spread COVID. But combined with other things like masking, trying to have good ventilation, all of those things, it’s one step for sure.

What is your sense of the current availability of COVID-19 tests in the United States?


SUSAN BUTLER-WU: We take it way back to the beginning. There was an enormous problem in this country with the availability of tests, and predominantly at that time, the only thing that was available was essentially PCR or PCR-like tests. Obviously, the capacity has increased, but I think one of the things that—if you look at the news this week, you see these big, long lines of people queuing up to be tested. And so I think what people need to understand, although, is we have way more capacity than what was there at the beginning, there is a cap to that capacity. Clinical labs and diagnostic labs are run by people. People are doing these tests, not, like, little pixies. And so there is a finite capacity with respect to supply, with respect to labor. And so it’s not an infinite pot. And so when we think about supply and demand, I can understand the frustration on the public—that they see, you know, delays. But those are because, all of a sudden, there’s this ramp-up in demand. That’s on the sort of diagnostic lab setting.

Obviously, with rapid tests, this is an area of some debate. There definitely are not enough tests right now. And obviously, today the president’s announcement that there’s going to be, I think, a half a billion tests that are going to become available to the public for free starting in January is an important step. But with that, we need to be very clear. I really think the government needs to be providing a lot of education around what test results do and do not mean and how to use them and how to interpret them for yourself.

What do you make of the efforts that the federal government has announced to make it easier to get tested for COVID-19?


SUSAN BUTLER-WU: Anything that we can do to make it easier for people to get tested is one part of the solution, right? But we also have to combine that with education, and I’m not hearing much discussion of that so far. So, it’s complicated, right? These are presented like they’re one test. They’re presented like it’s really—they’re easy to do. But unfortunately, interpreting the results in terms of the significance and how that affects your behavior can actually be quite challenging. So, for example, with any antigen test, how well it performs is also influenced by the prevalence in the population. So the time now where we’re sort of staring down the barrel of a surge that’s coming and we have increasing community spread, if you’re positive, you’re positive. Right? You may not even need to confirm it. But at a time where there’s low community spread, that could be a false positive. You might need to—you should get that confirmed.

The danger for me is that without appropriate education, if I have symptoms right now and I test myself, I should assume I have COVID for sure. But if I have symptoms, I test myself, it’s negative, it’s important that people understand that still means it could be COVID. So I think sometimes testing has been sort of held up on this pedestal, like it’s this singular solution to ending the pandemic, particularly rapid testing. And it’s not. But it is a key part of breaking those transmission chains to try to, you know, rustle this thing under control.

What do you think of “test to stay” protocols being adopted in some schools—where students with confirmed COVID exposures can continue attending in-person school, rather than isolating at home, if they get ongoing negative results on rapid tests?


SUSAN BUTLER-WU: So, let me preface this by saying I have school-aged children who are—one in particular—was absolutely devastating to stay home for a full year from school. So I think that keeping schools open isn’t just the highest of priorities, not—you know, obviously I’m biased as a parent. I will say, though, that it shouldn’t be done alone. The idea that you would do this instead of masking is, like, preposterous. We have to combine things. So I think if you are a school where your students are required to be masked and you are used—using a test-to-stay approach, I think it can be really helpful at allowing kids to continue to go to school while trying to reduce the risk of spread. I do not think it is appropriate to do it in place of masking in schools. Again, the name of the game here with everything to do with this pandemic is combining approaches and layering things. And so pitting one thing versus the other—like, I could have a test-to-stay instead of a mask—I think is where we’re going to fail. But combining those approaches, I’m very much in favor of.

Can you describe the pros and cons of different test approaches for COVID-19 (e.g., PCR, antigen, etc.)?


SUSAN BUTLER-WU: So PCR is pretty much the gold standard with respect to diagnosing infection. It is more sensitive than antigen testing, but the downside has been the turnaround time. So in a hospital setting, you know, if there’s a patient coming into the emergency department and they’re ill and they’re going to be admitted, that’s something that we can turn around really, really quickly in the hospital where I work—very, very rapidly because we’re testing for somebody coming in to be admitted to hospital, whereas when you go to your average drive-through, you’re not going to be getting an answer for the most part in a couple of hours. You’ll get your answer, if you’re lucky, the next day. And as the system starts to, you know, have increasing demand and increasing demand, that—those turnaround times can be protracted. And that right there is one of the major cons associated with PCR. So you have better sensitivity, but the con is waiting for the results. The flip side, then, is for antigen testing, where the pro is the rapidity of results. You get an answer in, like, 15 minutes. The downside is not being as sensitive, so not being able to detect as much of the virus as a PCR.

So where that comes into question is when we have to start to think about the two different reasons that you would test. So I have symptoms. And am I—I have symptoms, and I want to know whether this is COVID or not. That’s where the PCR, on a scientific level, is a better test ’cause it’s more sensitive. But if you’re not going to get an answer back for four or five days, it’s not as helpful. That’s where I think combining approaches is important because if you do an antigen test and you can get that (inaudible), great. But if it’s negative, then it doesn’t mean it’s not COVID, and you still need a PCR. The cons, obviously, for antigen are just that—the increased chances of false negative results and, on occasion, issues with false positivity.

How are you feeling generally about the holidays this year relative to last year?


SUSAN BUTLER-WU: I think we’re in a much better place than we were a year ago. So a year ago, we had, you know, just been vaccinated. None of my family members were vaccinated. A year later—right?—vaccinated, boosted, kids are vaccinated, have home rapid tests, have antivirals that are coming down the pike. So I feel like, although it is a nervous time and we—there’s a lot we don’t know about omicron, and certainly it’s—how infectious it is is kind of scary, I think we’re in much better shape than we were a year ago. And I feel—although I’m still anxious about the holidays, I feel like it’s definitely—we have tools to be a lot safer than we were a year ago when we were just limited solely to, you know, the mask and—which is still important. But the vaccines to me are key—big difference.

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