Placeholder Image

Subtitles section Play video

  • It seems intuitive that a more sensitive COVID  test is better than a less sensitive one. And yet,  

  • you may also have seen news stories suggesting  that some COVID tests are too sensitive,  

  • or that cheaper, faster (and even less  sensitive?) tests might be better. But  

  • why would they be better? And better for what? According to public health experts at Harvard  

  • and Brown University, in the fall of 2020 the  US needed to do 8x more testing to successfully  

  • suppress COVID-19, India needed to do 4X more  testing, Brazil needed to do 66X more testing,  

  • Bolivia needed to do 140X more testing, and  Mexico needed to do over 400X more testing.  

  • The idea behind sayingCOVID tests are too  expensive and too sensitiveis that part  

  • of what's kept countries from doing enough  testing is the kind of tests being used

  • The current gold-standard for COVID testing isnasal swab PCR [polymerase chain reaction] test.  

  • This test is highly sensitive, which means  it's able to detect the presence of the virus  

  • even at very low levels of viral load. That's  great for a doctor who wants to know whether  

  • their patient was infected with COVID-19. But if  you're screening an entire community for COVID  

  • outbreaks, then PCR tests have a few downsides. First, PCR tests are expensive. To avoid breaking  

  • the bank, many places are focusing on testing only  people who already have COVID symptoms or who were  

  • recently exposed to someone with COVID. Using  only expensive tests means that we don't test  

  • (and therefore, don't catch!) most people who  are contagious without showing symptomsand  

  • we know that a significant proportion of COVID  transmission comes from these asymptomatic people

  • Second, PCR tests take time: usually a couple of  days for results to come back from the lab. Here's  

  • what the typical viral load in an infected person  looks like over timethey're most infectious  

  • around the peak and their infectiousness declines  afterwards. So if test results take a few days to  

  • come back, chances are you miss this peakwhich  means missing the window of time during which the  

  • person is most likely to infect other people. Essentially, although PCR is very sensitive,  

  • it's also expensive and slow, so while it's great  for diagnosis, it may not be the best test for  

  • screening a community. And the cheaper, faster  testing alternatives areno surprisenot as  

  • sensitive as PCR. BUT - and this is a subtle point  and maybe wouldn't matter if PCR were cheap and  

  • fast, BUT when it comes to screening a communityPCR's high sensitivity could itself be a downside.

  • PCR tests can detect very low viral loads, which  is great if you happen to test somebody very  

  • early on in an infection, or very late and you  want to confirm whether they were infected. But  

  • this sensitivity also means that a person can  test positive for the infection when they're  

  • basically no longer infectiousand in fact, PCR  is so sensitive that it can detect viral fragments  

  • and give a positive result even when there are  no viable viruses left. So it's likely that by  

  • using PCR tests we're quarantining & contact  tracing some people when we don't need to

  • It's not that the PCR test is a bad test, in fact  it's a great test for what it's designed to do,  

  • which is to accurately and reliably detect even  very low viral loads. It's just that it's more  

  • expensive, slower, and perhaps even more sensitive  than would be ideal for screening a community

  • A less sensitive test could catch just  as many highly infectious people as PCR,  

  • but it would miss people who are post-infectiouswhich you could argue is a good thing for  

  • screening purposes since those people  are no longer a significant threat. The  

  • downside is that you'd also miss people who are  pre-infectious, which is undoubtedly a bad thing

  • But the math works out that if you have a test  that's just a little bit less sensitive than PCR,  

  • but you use it a lot more frequentlyyou're actually more likely to detect an  

  • infection compared to infrequent PCR testingSo can you identify more infections overall

  • All of this suggests that if you're trying  to screen a community for COVID outbreaks,  

  • you can get more testing bang for your  buck with rapid testing than PCR testing;  

  • that is, you can identify more infectious  people more quickly for less money.  

  • As long as the rapid tests are only slightly  less sensitive than PCR tests but a lot cheaper

  • Rapid testing is not a replacement  for PCR testing in a clinical setting,  

  • nor is it a replacement for masksphysical distancing, handwashing,  

  • and so on. And if PCR testing cost 1/10th  of what it does and took 1/10th the time,  

  • we probably wouldn't be having this discussionBut if we're going to test widely and frequently  

  • enough to suppress COVID (and we really  should), then we can get more for our money  

  • with tests that are cheaper, faster, and - perhaps  counter-intuitively - a little bit less sensitive

  • Ok, so that's the argument for rapid  testing, and here are the caveats

  • No medical tests are perfect, and a negative test  result isn't a foolproof guarantee of not being  

  • infected. Because rapid tests are less sensitive  than PCR tests, this makes it more likely that  

  • infected people will slip through the cracks  of a test. So as long as COVID is a problem,  

  • it's important to still take precautions  like wearing a mask and social distancing  

  • even if you test negative. And for  people who were likely exposed to COVID,  

  • health authorities recommend that they quarantine  for 14 days regardless of a negative test result,  

  • and that their negative rapid test  results are verified by a PCR test

  • When a rapid test is just a little less  sensitive than PCR, then the downsides of  

  • missing some infected people can be balanced out  by the benefits of more widespread and frequent  

  • testing. But this only works up to a point. If  the sensitivity of the rapid test is too low,  

  • the cost of missing many infections will  outweigh the benefits of widespread testing

  • The graph we showed for viral load was based  on measurements of patients' viral loads,  

  • and while all else being equal, a higher viral  load tends to mean you're more infectious,  

  • there are a lot of other things that  can affect how infectious you are 

  • We didn't really mention what happens when you  aren't infected but the test gives a positive  

  • result anywaythis isn't likely on any given  test, BUT if you're testing a huge number of  

  • people frequently, then just by statistics alone  you can end up with a large number of false  

  • positive results. The rate of false positives  is comparable between PCR tests and rapid tests,  

  • but matters more with cheap rapid tests  since the whole idea is to do way more tests

  • Finally, vaccines. So won't all of this  be irrelevant once vaccines come out?  

  • Well, vaccines will certainly help, but  unfortunately, testing will probably still  

  • play a major role in suppressing COVIDIt's really a topic for another video.

  • A very big thankyou to Brilliant  for sponsoring this episode of  

  • MinutePhysics - with MinutePhysics, I try  to bring you deeper level explanations,  

  • while Brilliant engages you to solve  fascinating, challenging problems yourself,  

  • guiding you to a deeper level understanding  of science and mathematics. Like their course  

  • on knowledge and uncertainty - what's the most  efficient way to zero in on the true criminal  

  • from a lineup of suspects? Or, how worried  should you be if you know a medical test is 90%  

  • sensitive and you get a positive result? To improve your knowledge and reduce your  

  • uncertainty, go to brilliant.org/MinutePhysicsand sign up for free. The first 200 people will  

  • get 20% off the annual Premium subscriptionwith full access to all of Brilliant's courses,  

  • puzzles, & daily challenges. Againthanks to Brilliant for their support.

It seems intuitive that a more sensitive COVID  test is better than a less sensitive one. And yet,  

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 pcr test testing sensitive covid rapid

The Case for LESS Sensitive COVID Tests

  • 2 0
    Summer posted on 2021/01/14
Video vocabulary