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Science and Innovations

Weekly Update With Dr. Paul Keim: Testing For COVID-19

Josh Biggs/NAU

Scientists say widespread testing for the coronavirus disease is vital before public health officials can make decisions about reopening the economy. Multiple tests have been designed for the new disease, but supply shortages now hinder the effort. In KNAU’s weekly update on the science of COVID-19, Melissa Sevigny speaks with infectious disease expert Dr. Paul Keim about how testing works.

Melissa Sevigny: Current tests for the coronavirus disease rely on testing the virus’ genetic sequence. Can you tell me how that works?

Paul Keim: The gold standard in novel coronavirus testing is based on a process called PCR, or polymerase chain reaction. This is a method that is just exquisite in its ability to detect very small numbers of genetic material… The CDC and a host of others, actually, have developed assays now where they can focus in on one portion or several positions in the viral genome, and then even with as little as a single molecule they can amplify that up into millions and millions which then can be interrogated for whether that virus was present or not. So PCR is the best method for detecting the virus and certainly the most common test at this point in time.

So what other kinds of tests are available or being developed for this disease?

There’s tests that are becoming very common, and you’re going to see lots and lots of these in the next week or two, called serological tests. These tests look for antibodies in your blood that your body produced in response to the virus. That test isn’t as useful for early diagnosis…because it takes your body a couple of weeks really, maybe a week or two at the very earliest, to produce those antibodies. One you start producing them, then this test can detect that, and that can be useful in diagnosing some patients, especially if they’ve been a little bit tardy in getting to the clinic, but it is really useful for telling you if you’ve had the disease or been exposed to the virus in the past. That’s a type of test that we’re relying on in the future to really understand what herd immunity might be and how much disease has been in the community that we didn’t even know existed.

Herd immunity, what exactly is that?

Herd immunity is when there’s enough people immune to the virus that it can’t sustain an epidemic, it can’t sustain an outbreak…. The estimates vary, but somewhere between 40 and 60 percent of the population needs to be immune to stop this virus.

What do we need to know about the accuracy of these types of tests?

All tests have weaknesses. Even the gold standard PCR test has weaknesses. What we really need to know is what their false positive rate is and what their false negative rate is…. For example, the PCR test that’s used… it probably has no false positives. The genetic material of this virus is uniquely different from its all of its closest relatives… On the other hand, when you have the disease, you don’t shed virus at the same rate all the time. The false negatives, then, from this test are really due to the fact that there are some people who have this disease but are shedding small amounts of virus.

Talk to me about why it’s important that we’re able to roll out widespread testing for this disease.

Of course as you know, Melissa, right now the state of Arizona is in a lockdown situation. You and I are talking from our bedrooms or someplace. We’ve got to get past that... In order to do that, we’ve got to know how many people are infected, who has active infections… Those types of things have to be determined in a very systematic fashion for us to get to the point where we can start to open up our society.  So the testing is important for people who are sick, so that physicians and healthcare workers can start to treat them appropriately … but these public health policy decisions are going to be key to getting our culture and our society moving again, and those folks really need good data in order to make those decisions.

Paul Keim, thanks for the update and stay well.  

Thank you Melissa.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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