As Wyoming slowly begins to reopen, the Department of Health says widespread testing is critical. In the simplest terms, the COVID-19 test works by taking a sample from a potentially infected individual, and it's scanned for the virus. But it turns out like any test, the COVID-19 test is not 100 percent accurate. Wyoming Public Radio's Kamila Kudelska spoke with University of Wyoming Associate Professor of Community and Public Health Dr. Christine Porter about why it matters that there is a possibility of false negatives.
Kamila Kudelska: How can these false negatives happen? In Wyoming, we're a large state. These samples are being transported for a long time and stuff like that. So does that factor into those potential false negatives?
Christine Porter: So, let me just say everything about the coronavirus is a little bit still up for grabs. We're increasing the science and our knowledge. But there's still so much we don't know. I'm going to assume with good reason, I think that people know what they're doing in terms of transporting the samples and in running the labs. It seems that the main source of false negatives, like you actually have it, but the test doesn't show you it, is that we're not able to get a good enough sample of the body fluids that actually contains the virus that's in your body. So almost all the samples, if not all of them in Wyoming, are to my knowledge, sticking the swabs deep up your nose, and doing a swab up at the top, high up in your nose. I understand we have a shortage of swabs, at least sometimes in some parts of the country. And it's dangerous for the person taking the sample, right, if the person is positive, sticking that up their nose, they're up close and personal and can make them sneeze and cough, which obviously can spread the virus. So a lot of personal protective equipment is needed on the part of the sampler in addition, so it's not an easy process. And your viral load, if you have the virus, but it's a relatively low
level of infection, you have mild symptoms, maybe even none, it's even harder to pick up the virus. But if you don't get it on that sample, it's going to test negative even though the virus might be in your body. So mild cases are less likely to show up positive, because they're less likely to catch it on the sample that they take. Also cases that are farther out from infection, the infection level, the amount of virus in your body might be decreasing. And so those cases might also give you a false positive even though they have a virus and may be spreading it.
KK: So I know that there has been some recent research out there that gives a wide range of sensitivity for those tests.
CP: Yes, there are almost no tests that are 100 percent sensitive, right? Perfection is not a thing, especially in biology. Probably at most, these tests are like 95 to 98 percent sensitive. Let's say that our tests are 95 percent sensitive. That means that only five percent of tests will have a false negative. That's actually a decent sensitivity, right? That would mean that among the almost 9,000 negative results we've had so far in Wyoming, we've done over 9000 tests [but] 404 so far have been positive, and 8,895 tests have been negative. That implies that five percent of those are not actually negative of those 8,895. That would give us a total case rate that's way more than double the 404 just among the people we have tested so far, who have had negative results. Some of the studies coming out that have compared methods of sampling and testing have found sensitivity rates that are as low as 72 percent for the nasal swabs, which means 28 percent of tests would have false negatives. That implies we have actually nine times as many cases as the 404 we currently have listed. We don't know for sure what our and how well we're doing with that.
KK: Why should Wyomingites care about the fact that there is this possibility of the sensitivity being pretty high? Or even just that there is a sensitivity, you know that 5 percent rate, 95 percent?
CP: A few reasons. One is at the individual level. Even if you were one of the few people to get a test, and it comes back negative, you can't be sure you're negative. If you have symptoms, you should assume you actually do have COVID and quarantine accordingly and contact trace accordingly. And the other issue at a more public health level, we've done a phenomenal job of social distancing and doing other prevention measures. But as soon as they start to lift, the spread starts to go up. And the more people we have who are infected, the faster that's going to happen. And so if we're making public policy, assuming we only have 400 people who have it, instead of 2,000 or 3,000 people who have it, then we lift social distancing and other restrictions too soon. We're going to be really fast into a fire that's raging way higher than what we were able to prevent.
Have a question about this story? Contact the reporter, Kamila Kudelska, at kkudelsk@uwyo.edu.
