Testing for COVID-19 at Home

John Whyte, MD; Sean Parsons, MD


September 01, 2021

Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

JOHN WHYTE: Welcome, everyone. I'm Dr. John Whyte, the Chief Medical Officer at WebMD. I'm glad we're talking about vaccines. And that's an important step of controlling the COVID pandemic. But we're starting to forget about testing, and the need for testing. And now we have opportunities to test at home. But should you do a home test? How accurate are they? And does it matter whether or not you have symptoms? When do you test? So to help unpack this and answer the questions that are on your mind, I've asked Dr. Sean Parsons. He's the CEO and founder of Ellume Health. Dr. Parsons, Thanks for joining me.

SEAN PARSONS: Hi, John. Good to be with you.

JOHN WHYTE: Well, let's start off with, let's explain the technology a little, because we talk about rapid tests, we talk about PCR tests. They test different things and tell us different things. So can you help explain to our audience, where we're not giving them an immunology lesson, but we're differentiating rapid tests and PCR tests.

SEAN PARSONS: So antigen tests, such as ours and most of the products that are on the market, detect a viral protein called the nucleocapsid protein. That's one of the internal proteins of the coronavirus structure that's associated with replicating viral RNA. That protein is very highly conserved, meaning it doesn't change very much through these different strains, and it gives us confidence that by detecting that protein, we get an indication that there is virus present in the sample.

Within the antigen test, there are a couple of different categories. There's the visually red tests, which is the two lines for positive, one line for negative products. And then there are the fluorescent immunoassay products, which have higher sensitivity based on using fluorescence to detect small amounts of the coronavirus protein of interest. Those antigen tests are different from the molecular tests, or the PCR tests, which are about amplifying up a small piece of viral RNA to identify very, very small amounts of virus present in the sample.

JOHN WHYTE: They're looking for genetic material as opposed to protein. And that may result in different levels of accuracy. So let's spend a little bit of time. I do want to point out to everyone that both type of tests, the antigen test and the molecular test, or what we're calling rapid tests and PCR tests, are available. Both of those are available at home.

The PCR test, you're still going to have to mail back. Those are the only tests currently that right now are available in the United States. And then the rapid test, obviously, you can do at home. Or you could do either of them at your local pharmacy in many areas. But let's talk about the accuracy, because they work differently in terms of detection. Let's start first, if you don't mind, with the PCR test, because you kind of hold that out as the gold standard, so to speak.

SEAN PARSONS: They're really the best-accuracy products that are available at the present time. And so those products are reasonably consistent between them. There are small differences between the tests. But for all intents and purposes, the PCR tests I refer to as the gold standard or truth.

JOHN WHYTE: So if you test positive, it's a high likelihood you're positive. If you test negative, high likelihood that you're negative. Let's move to the rapid test, the antigen test. Talk to our audience about the accuracy of these tests.

SEAN PARSONS: Some of the rapid antigen tests, particularly those that use fluorescence detection, do generally have better accuracy, particularly sensitivity, than the other tests. Now sensitivity is being able to correctly identify the people that do have COVID. So catching all of, catching those positive cases. Our test in the clinical study we ran for the FDA showed a sensitivity of 96%.

JOHN WHYTE: Let me ask you: The population you studied for FDA, was it symptomatic, asymptomatic, or both?

SEAN PARSONS: So that was both symptomatic and asymptomatic. In the symptomatic population, our accuracy is 96%. In the asymptomatic category, our sensitivity is 90%. So those people who were asymptomatic are shedding a little bit less virus and we have slightly lower sensitivity than compared to, that's 90%, as compared to PCR.

JOHN WHYTE: So what should people do if they take a rapid test and they get a negative result? How do they interpret that in terms of their activities? Let's start with if they're symptomatic. Should they wait a couple of days and do another rapid test? Should they go to PCR? Or should they say it was negative, I'm done?

SEAN PARSONS: So people who are symptomatic and who have a negative rapid antigen test should be viewed as having a low probability of having COVID at that point. That the rapid test on symptomatic people is quite good. So I think at that point, they should have a high degree of confidence that they don't have COVID. If their symptoms don't improve or their symptoms deteriorate, then repeat testing would be justified.

They could choose to test with another rapid test, or they could choose to escalate to a molecular test. So that would be up to people to say, look, the first test is negative, the chance of me having COVID is low, but if things don't improve or they remain concerned, then repeating that test is something which we recommend, and so do the other manufacturers.

JOHN WHYTE: What about if you're symptomatic and you test negative? There could be an argument that there's not enough viral load. Either the sample wasn't as good as it should be, or that you yourself just not have developed enough virus for detection and that you should retest in 2 days. What's your thoughts on that?

SEAN PARSONS: We certainly are of the view that taking a reliable sample, taking a high-quality sample, is very, very important to getting a reliable result. And we at Ellume have created a sample integrity control, which is built into our test. So if you don't take a sufficient sample or you don't take any sample, then our test will give you a sampling indeterminate result. Because we don't know, is the answer. We don't know if you've got COVID or not if we don't get enough sample to be able to detect it.

Ellume is actually the only of the rapid antigen tests to have that sample integrity control built into our test. And we think it's very important, particularly in the hands of a lay user. For us, that gives the user, and the clinician who might be involved in this, even more confidence that a negative result can be trusted, that a negative result has had a reliable sample collected with it, and that they can have confidence that the probability of having COVID is low.

It's not zero, it's not zero. And so where that clinical risk exists or where people are worried and anxious that they could have COVID, then repeat testing would be a worthwhile thing to do. Either to use our test or another rapid test, or if they're particularly anxious or in the high-risk category, then to escalate that up to the next level and to have a molecular test. It will be infrequent, but it will, from time to time, there will be the odd case that is missed in that very first test.

JOHN WHYTE: So what do you tell the viewers who are listening, have Delta surging in many areas of the country and around the world: Who should take an at-home test, and when should they do that test?

SEAN PARSONS: That is based on people's risk of exposure. Obviously, if you're living with someone who's just been diagnosed with COVID, then your probability of exposure is actually quite high over a course of some time, and we would strongly recommend that those people seek to identify their COVID illness and to be confident that they don't contract COVID. And we certainly think that rapid antigen tests have an important place in that loop.

JOHN WHYTE: Well, Dr. Parsons, I want to thank you for taking the time to help explain how rapid tests work and how we interpret the results. That's important information for folks to know right now while we're still in the middle of the COVID pandemic.

SEAN PARSONS: Thank you, my pleasure.

JOHN WHYTE: And if you have questions about COVID, drop us a line. You can email me at Dr.John@WebMD.Net. Thanks for watching.

This interview originally appeared on WebMD on August 31, 2021

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