Coronavirus testing alone isn’t enough to ensure safety during the holiday season, experts say

Paula, who works for a federal agency in the Washington, D.C., suburbs, has a carefully orchestrated plan to see extended family for the holidays ahead. A test for the coronavirus is only one small element.

Paula — who asked to be identified only by her first name to protect her family’s privacy — her husband, and their two kids went into quarantine recently after her youngest was exposed to the virus at preschool. The entire family was tested a week and a half later, and all tested negative, but Paula plans to have her son retested again after a full two weeks.

Even so, she's not relying on tests. The entire family will complete a strict two-week isolation before seeing family for the holidays, with the kids continuing virtual learning until next year and both parents working from home.

A test alone cannot provide the information you need to decide if you can safely see family outside your household, experts say. A positive test result is usually pretty reliable, but a negative result is a lot trickier to interpret.

Tests have become much more common in the past few months. In late November, right before the Thanksgiving holiday, the United States hit 2 million coronavirus tests a day for the first time. And they are an important part of controlling the pandemic — you can't stop transmission of a virus without first knowing who has it.

But it's important to know what the tests cannot do as public health officials warn about catastrophic increases in infection levels and the perils of letting your guard down during the holiday season. A test doesn't mean you can ignore the need for protective measures such as wearing masks, avoiding large indoor gatherings and quarantining.

A negative result doesn't always mean you don't have the coronavirus. You could have been exposed but have not yet developed enough of the virus for a test to detect, for instance. And you can still be contagious even if you test negative.

» READ MORE: CDC recommends people wear masks indoors when not at home

There are a few different types of coronavirus tests, and they have different levels of accuracy, especially when it comes to determining whether you are virus-free. But even the most accurate "gold standard" of tests, called PCR (polymerase chain reaction), can frequently give false negative results, research shows.

The problem with false negatives can occur when test samples are collected. One method collects a sample from the back of the nose and throat, called a nasopharyngeal swab. While less invasive samples, such as throat swabs or nasal swabs that don't go as far into the nose, are becoming more common, nasopharyngeal swabs are usually the best at finding the virus where it often lives — deep in your nasal cavity.

But even the nasopharyngeal swab may not collect all traces of the virus — perhaps because it wasn't inserted far enough, or because the infection has already moved to the lungs or elsewhere in the body. Or you might not have enough virus to shed it — either because you're early or late in the infection.

The sooner the test is performed after exposure, the higher the chance of receiving a false negative result, according to an analysis published in the Annals of Internal Medicine. There's a 100% chance you'll test negative on the day of exposure, but it drops to about 38% after five days, around the time symptoms usually set in, and then falls to 20% three days after that.

"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," said Lauren Kucirka, a resident physician at Johns Hopkins Hospital and lead author of the study. The analysis does have some limitations, with wide confidence intervals that mean the percentages mentioned in the study can vary quite a bit in real life. The study also didn't look at false negatives among those who never developed symptoms.

If you haven't developed symptoms, it can be particularly difficult to rule out infection.

That's because the tests are designed for symptomatic people, and without symptoms, it's hard to know how long you've had the virus — which means you might be tested too soon or too late after exposure to get an accurate negative.

This is particularly important because nearly half of people never develop symptoms, yet they are responsible for the majority of transmissions, the Centers for Disease Control and Prevention said recently.

» READ MORE: Some Philadelphians face weeks-long delays for COVID-19 test results through urgent cares, pop-up sites

What's more, Muge Cevik, a clinical lecturer in infectious diseases and medical virology at the University of St. Andrews, said, "We don't yet understand exactly when a person who's infected will start testing positive for the virus. So there are situations when a person could test negative, but they could still be contagious."

Usually, it takes about four to five days after exposure for the viral load to peak and for symptoms to set in, and the viral load is highest for the next five days — which tracks with Kucirka's finding that this period is the most accurate time for tests.

But the most important finding of that study is "there's never a time that you can rely on it 100%" to know whether you're negative, Kucirka said.

Alex Zorach had several common COVID-19 symptoms in March — an elevated temperature just shy of a fever, a slight cough, shortness of breath, chest pain. After two days of intense breathing difficulties, Alex drove to the nearest emergency room in Christiana, Del.

In the ER, though, a nasopharyngeal swab was negative.

The ER doctor said the test result should be treated as a false negative, because all of the symptoms pointed to COVID-19. Zorach isolated at home and successfully avoided spreading the virus to colleagues and friends.

After months of debilitating symptoms even after a relatively mild illness, Zorach had a simple message: "Be warned, people. If you get a negative test, it doesn't mean you don't have it."

Another testing option, antigen tests, which look for proteins from the virus, also use swabs in the nose but are faster and less accurate than PCR tests, which are sent to labs to detect the virus's genetic material.

In addition, blood tests can look for antibodies after an infection, but these tests are not used to diagnose illness and tend to be even less accurate.

Anne Wyllie, an associate research scientist in epidemiology at Yale School of Medicine, says that PCR tests now are more accurate than early studies like Kucirka's indicated, but she still cautioned against making decisions based on one negative result.

"It's really important that people don't fall into the trap of, hey, I've had a negative test. I'm OK," she said. "You do not just want to rely on one test result."

This is especially true of the less-accurate antigen tests, she said. "They could be negative in the morning, but because these tests are less sensitive, they could actually be peaking with contagious virus load that evening."

Watching for symptoms, especially mild symptoms such as back aches or fatigue, is also important.

“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.”

Other experts agreed. "No test is absolutely perfect," Cevik said. "If there's high clinical suspicion for the infection, we often repeat the test."

» READ MORE: Philadelphia's current COVID-19 guidelines

And if you think you have been exposed, take all possible precautions and isolate for at least 10 days, said Rangarajan Sampath, chief scientific officer at the Foundation for Innovative New Diagnostics.

"Other than getting multiple tests negative," Sampath said, "you can't really assume you're negative."

Those traveling for the holidays need to isolate after their journey in case of exposure on the way, said Claudia Finkelstein, an associate professor of family medicine at Michigan State University. She traveled to Montreal this year to see her parents. She flew there and isolated in a rental home for 14 days after arrival before she was confident she could see them safely.

“When meeting with family, or friends,” Cevik said, “we just need to assume we may have COVID-19, and change our behavior accordingly.”

Paula had originally planned to celebrate Thanksgiving with her extended family after quarantining for five days and testing negative once but she canceled those plans because they were not safe enough, especially for her parents, with the coronavirus spreading rapidly.

Although her new plans for the holidays ahead are more work, she said, they are unquestionably worth it.

“I want my parents around for as long as humanly possible,” she said.