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Health

Is a Negative COVID-19 Test Result Truly Negative?

If your coronavirus test came back negative, you might not be out of the woods just yet. The trustworthiness of a negative result depends on the type of test you took, when you tested, and how much contact you've had with other people.

By Leslie NemoNovember 9, 2020 7:20 PM
relief coronavirus deep breath woman take off mask - shutterstock
(Credit: Elizaveta Galitckaia/Shutterstock)

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Celebrities have definitely done it, and maybe you’ve done it too: Get a COVID-19 test and if it’s negative, go on vacation with your closest friends.

Experts warn that this is not a good idea. That's because a negative COVID-19 test only says so much. Whether the test correctly identifies you as uninfected depends on the kind of test you were given, how well the test was administered, and what you’ve been doing (and who you’ve seen) recently.

So, a negative test result by itself is not a “get out of jail free card,” says Mary K. Hayden, the chief of the division of infectious diseases at Rush University Medical Center.

Which COVID-19 Test Is Best?

For starters, there are dozens of COVID-19 tests out there, and each technology comes with different likelihoods of providing an incorrect diagnosis. False negatives — or a test that says you don’t have the virus when you actually are infected — are more common with some types of tests than others. False positives, on the other hand, tend to be more rare.

The gold standard coronavirus test is the PCR test, says Tom Russo, an infectious diseases physician at the Jacobs School of Medicine and Biomedical Sciences. PCR tests, which are typically administered by hospitals and large commercial labs, are the least likely to give a false negative result. This test relies on finding trace amounts of the virus' genetic material in the test sample. If present, the test copies the viral genome repeatedly, essentially serving as a megaphone that can amplify even a subtle presence of the virus. If there's a very low-level infection present, the PCR test is the exam that is most likely to pick up on it.

Besides the PCR exam, there are also COVID-19 rapid tests. As the name implies, these turnaround faster results and are common in non-hospital settings like nursing homes and university campuses. The speed of these tests often comes with tradeoffs. Some rapid tests are like PCR in that they work by finding bits of the coronavirus genome but use different techniques to amplify those molecules. So far, the available scientific evidence suggests that rapid tests are more likely to read out a false negative.

Another rapid option is antigen tests, which look for fragments of coronavirus proteins. Though relatively cheap compared to other tests, available evidence with these nose or spit tests also indicate they're more likely to produce false negatives.

Another factor that can cause an issue with test reliability is human error. If a nasal swab doesn't get deep enough in someone's nose, it might not produce a reliable sample. Another misstep that can impact the reliability of a coronavirus test result is letting the spit or mucus sample sit around too long before it's processed, which causes the virus' genetic material to degrade, Russo says. 

But generally speaking, it's advised to leave test administration to the professionals for now. Self-tests are certainly convenient, but they're less accurate for a number of reasons. Spit tests are common for at-home tests, but come with some downsides. And people at home (understandably) will have a difficult time shoving a swab into the far depths of their own nose.

The Social Factor

Even if a COVID-19 test comes back as negative, you also have to consider your social situation. Who you have interacted with recently, and how prevalent the virus is in their community, also matters for COVID-19 test trustworthiness, Russo says. For example, maybe someone comes down with the classic first symptoms of COVID-19 — a loss of smell and taste — a few days after they attended an indoor Halloween party without masks, and another attendee was positive for coronavirus. If that person's test comes back negative, they should be suspicious of that test result, Russo says.

But even social scenarios that don't have all those red flags can color your test results. Most people who are socially distancing and taking other precautions still might have to do things that come with some level of coronavirus infection risk — like going to the grocery store, the dentist, or to work.

"It's not like you're a hermit up in the North Canadian tundra," Russo says. "If you've been up there for a month and you get the negative test — you're pretty good, right? But unfortunately, most of us are constantly interacting with other individuals, and it presents a difficult situation." If someone lives in an area where the case count is high and has been pushing the boundary when it comes to risky behaviors, a negative result on one of the faster, less-trustworthy tests warrants a re-test using a gold standard option, Russo says.

One Test Is Not Enough

A negative test result can be reassuring, but be careful about putting your full confidence in it. Although a negative test result "has some value, it has limited value," Russo says. "You have to realize it's just a single point of time, and that person could be positive the next day or the day after that, as the incubation period plays out."

Russo explained that it can take days from when you’re actually exposed to the coronavirus to the point at which it’s built up enough in your body for even the gold standard tests to detect. Most people who caught coronavirus will test positive six days after exposure, and nearly everyone who's infected will test positive within 14 days of exposure. So, for someone who goes to that indoor Halloween party and gets tested the day after, they aren’t in the clear. It’s possible there hasn’t been enough time for the infection to build in their body. 

Medical professionals put faith in consistent testing that tracks someone's coronavirus status over time. Regular tests can show patterns in results, which makes it easier to detect the onset of an infection. This concept helps explain why you might have heard a lot of medical professionals advocate for frequent and widespread COVID-19 testing. “If we could do daily testing with the rapid turnaround, that would really be helpful, and then we package that together with a message from up top that everyone should wear masks and physically distance,” Russo says. “Then we can get a grip on this.”

These uncertainties might not be easy to hear as we head into the holidays, when you might want to get cleared by a COVID-19 test and see your family. But it's important to avoid a false sense of security. So, if you get a negative result, maybe you can breathe easier — just keep the mask on while you do so.

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