When to get a Covid-19 test: 3 things to know before traveling
Here's how to get the most out of your Covid-19 test.
Traveling home for the holidays is fraught in the best of times, but add Covid-19 to the mix and it becomes even more complicated. In 2020, in addition to arranging travel plans — or even small talk plans — you may also be making testing plans.
As of November 11, the 7-day rolling average for new coronavirus cases in the U.S. stands at over 128,000. If traveling is avoidable, the rising case counts across the country are a good enough reason to skip it, says Steve Woloshin, a general internist and co-director of the Center for Medicine and Media at Dartmouth College. He’s also the author of a recent New England Journal of Medicine paper on the implications of false negatives in Covid-19 testing.
“I think with travel right now, it's probably better not to, if you can avoid it,” Woloshin tells Inverse.
But if travel is still going to happen, it must be paired with testing.
Unfortunately, it is impossible to completely rule out the chance of a false negative test (meaning the results come back clear, but you still have the virus). To get the most out of a coronavirus test, there three key factors to consider:
- The type of test
- The interval between when you may have been exposed to the virus and when you get tested
- If your own behavior and local Covid-19 levels suggest you should actually believe that test result
The difference between coronavirus tests
There are two types of diagnostic tests for coronavirus, which means they indicate a current coronavirus infection.
There’s the PCR test, which measures the amount of virus genetic material in the body. These results are typically returned over the course of a few days. There’s also the antigen test, sometimes called a rapid test. These tests are often returned the same day.
The PCR test, conducted via nasal swab, is the "gold standard" for Covid-19 testing.
There are different metrics used to evaluate test accuracy. Travelers are probably most concerned about the test accurately reading positive if they do indeed have the virus — that’s captured by a metric called sensitivity.
The FDA reported sensitivity for all coronavirus tests should be taken with a grain of salt, Woloshin explains. The verifications required under the Emergency Use Authorization aren't robust enough to truly demonstrate how accurate they are in real-life situations. For asymptomatic people specifically, the data we have currently can't tell us how truly accurate the tests are.
With that in mind, PCR tests generally have higher sensitivity compared to antigen tests, which means that they’re more likely to accurately pick up a case of coronavirus. They’re the “gold standard.”
“The general consensus is [antigen tests] are less sensitive and more likely to miss an infection,” adds Woloshin.
When to get tested for coronavirus before traveling
If you get a PCR test the same day you happen to be exposed to coronavirus, the odds of that test turning up an accurate, positive result are almost zero. That’s according to an analysis of seven studies of Covid-19 PCR tests published in August in The Annals of Internal Medicine.
Woloshin says that’s the best paper he’s seen on the topic so far. He also says that antigen tests conducted on the actual day of exposure may not catch an infection.
Lauren Kucirka is the first author of that paper and a resident at Johns Hopkins Medicine. Her study showed that on the day of infection, there was a 100 percent chance of receiving a false-negative test result. By day four after exposure, the chances decreased to a 67 percent chance of a false negative.
“If you have a known exposure, it's reasonable to wait five to seven days after the exposure [before testing],” she says. “
But because the coronavirus can be spread asymptomatically, most people don’t really know when they were last exposed to the virus. That means you don’t know if you are on day one post-exposure or day five or seven.
There's where quarantine comes in.
If you do plan to use a test to travel, both Kucirka and Woloshin note that it’s best to quarantine before getting tested. Assuming that quarantine will end potential chances for exposure, that starts the clock ticking on that 5-to-7 day window, Kucirka explains.
“If you're quarantining then you know you haven't had an exposure. And if you were infected, you're not day one, two, or three. If you've been quarantined you know that you might be more day six or seven,” she says.
Quarantining before testing allows for the incubation period of the virus – which is the time before symptoms show in which the virus begins to replicate in the body. This period could last as long as two weeks, however, the CDC notes it is typically four to five days. Eventually, the virus may replicate enough to be detected by a test.
“I think it's reasonable to assume that your chances of testing positive are positively correlated with your viral load."
Because asymptomatic people do test positive for coronavirus, this suggests there is enough virus in their bodies to be detected. However, we still don’t know much about what the virus is doing inside the body after exposure — and why it can eventually cause asymptomatic people to test positive, says Paul Wikramaratna. He's the first author of a recent preprint paper (not peer-reviewed) examining the accuracy of PCR tests in symptomatic individuals.
Wikramaratna holds a degree in mathematical biology and until 2015 was a post-doc at The University of Oxford. Now he holds a job in the private sector.
“I think it's reasonable to assume that your chances of testing positive are positively correlated with your viral load,” Wikramaratna tells Inverse. “In asymptomatic people, I think it's reasonable to assume that that still holds, but what does the curve of what the viral load looks like over time?”
That’s all to say that pinning down when asymptomatic people are the most likely to test positive remains an open question.
And after all of this, there’s still no changing the fact that the test could still turn up a false negative, regardless of our best intentions.
Should you really believe the test result? – Kucirka says that the accuracy of the coronavirus tests we currently have are in line with the ones we use to diagnose the flu.
“We know that we get a lot of false-negative flu results, too,” she explains.
"It's just never been as much of an issue because we've never been in the setting of a pandemic.”
Quarantining before testing can help you know the number of days that have elapsed since you've been exposed to the virus.
In light of that, you can use your own common sense to judge whether a negative result accurately reflects what’s happening inside the body. That means asking yourself the tough questions.
What’s the Covid-19 positivity rate in your area? For example, in hot spots like the Midwest, the odds of encountering the coronavirus are simply higher, says Woloshin. Have you been avoiding crowds? Wearing a mask? Meeting people inside? Do you have symptoms?
Receiving a negative test result is one thing. Believing a negative test result is another.
If all the signs point toward you likely being exposed to the virus, a negative test result shouldn’t embolden anyone to discount that reality. (Just look at the White House super spreader event at the Rose Garden.)
Visiting family doesn't have to be completely ruled out. But knowing the downsides of visiting with a false negative test can help put things in perspective. There’s the regret of missing out on a holiday with family. There’s also the regret of putting a loved one at risk.
“If you do go to visit family and stuff, you have to balance the two regrets,” Woloshin says.
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