Across the country, Americans are weighing whether to travel and gather for the holidays—but public health experts warn that while coronavirus tests should be part of any travel plans, a test alone does not mean you can safely visit friends or family.

The 3 biggest questions about Covid-19 testing, answered

Why testing isn't reliable

While a negative coronavirus test result may come as a relief to some, health experts warn that testing inaccuracies and research limitations mean a negative test result might not actually prove you're not infected.

Research shows that even the RT-PCR test, which is considered the most accurate method for detecting whether a person is infected with the new coronavirus, frequently delivers false negatives.

Experts believe that's because of the way the virus travels within the body. For instance, experts say individuals may test negative because their infection has moved from their nasal cavity to their lungs or somewhere else in their body, which means a test designed to collect samples from the back of a person's nose or throat may not detect the virus.

An analysis published in August in the Annals of Internal Medicine found the likelihood of receiving a false negative increases the closer people are tested to their exposure date. Specifically, according to the study, the likelihood a person will test negative for the coronavirus drops from 100% on their first day of exposure to 38% after five days of exposure and 20% after eight days of exposure.

"It seems like around the time of symptoms, or a few days after, is when you're most likely to get an accurate [negative] result," Lauren Kucirka, a resident physician at Johns Hopkins Hospital and the study's lead author, said.

But at this point it's all a guessing game. Muge Cevik, a clinical lecturer in infectious diseases and medical virology at the University of St. Andrews, said researchers aren't yet entirely sure "when a person who's infected will start testing positive for the virus," which means "there are situations when a person could test negative, but they could still be contagious."

According to Tom Taylor, a professor and former statistician at CDC, the agency under normal circumstances would conduct a study to determine the accuracy of tests. However, that research could take up to a year—and when pressed with expanding America's capacity to test people for the new coronavirus amid the country's growing epidemic, CDC didn't research the tests as robustly as it typically would.

In addition, FDA in response to growing demand for the tests relaxed regulations that ordinarily would have required testing manufacturers to submit applications for the new tests and go through FDA's typical approval process. The change allowed labs to start marketing and using the tests in the United States without going through FDA's formal review process, and instead requires the labs to submit internal research on the tests' safety and accuracy at a later date.

As a result, Bill Miller, a physician and epidemiologist at Ohio State University, said the majority of the tests were "going through a really rapid validation process" and, "[a]s a result, we can't be completely confident in how they will perform."

The tests also may be less accurate in people who have contracted the new coronavirus but are not experiencing symptoms of Covid-19, the disease caused by the virus. Emily Landon, a hospital epidemiologist and infectious diseases specialist at University of Chicago Medicine, said, "We don't know how good these tests are in individuals who don't have symptoms." She explained, "We know they're pretty good at picking up [the virus] when it's present in people who have symptoms. But we have no idea what a negative test means in an individual that doesn't have symptoms."

Some people may also receive false negative tests if their viral loads have not yet climbed to the point of detection. It usually takes four to five days for a person's viral load to peak after their initial exposure to the coronavirus—and their viral load remains at high levels for the next five days, according to the Washington Post.

How you should respond to a negative coronavirus test

In light of the issues regarding testing accuracy, health officials and experts warn that receiving a negative test result doesn't necessarily mean a person isn't infected with new coronavirus.

Experts say people should remember there's a possibility they've received a false negative test because their viral loads haven't risen to detectable levels. It's also possible for people to have contracted the coronavirus after they underwent testing, the New York Times reports.

FDA cautions that "[n]egative results must be considered in the context of an individual's recent exposures, history, presence of clinical signs and symptoms consistent with Covid-19."

"A negative result is a snapshot in time," said Paige Larkin, a clinical microbiologist at NorthShore University HealthSystem in Chicago, where she specializes in infectious disease diagnostics. "It's telling you that, at that exact second you are tested, the virus was not detected. It does not mean you’re not infected."

Ashish Jha, dean of Brown University's School of Public Health, said, "I don't want somebody to have a negative test and think they can go visit grandma."

As a result, experts recommend people take all possible precautions to protect themselves and others against the coronavirus, including wearing face masks, maintaining physical distance, limiting contact with people outside of their households, and isolating after travel.

For instance, Claudia Finkelstein, an associate professor of family medicine at Michigan State University, who flew to Montreal earlier this year to visit her parents, said she isolated in a rental home for 14 days once she arrived at her destination to confirm she hadn't become infected with the coronavirus during her journey and ensure she could safely see her parents.  

Kucirka said it's also important for people to monitor their symptoms. "I wouldn't want people to be falsely reassured and ignore worsening symptoms because they had a negative test," Kucirka said. "Even if they test negative—if they develop symptoms, they should be highly suspicious for an infection."

And despite the limitations of testing, experts still recommend people try taking a coronavirus test before they visit their family or friends. Experts say people should aim to take more than one test leading up to the visit, including on the day they're planning to see a person who is at risk of develop a severe case of Covid-19.

As Cevik said, "When meeting with family, or friends, we just need to assume we may have Covid-19, and change our behavior accordingly" (Schreiber, Washington Post, 12/5; Parker-Pope et al., New York Times, 12/9).