If the self-test is negative, do I still have a chance of having Covid?

What does a negative result on a Covid-19 self-test really mean?

This is the question that confused many people who took the test at home because they had a sore throat, cough, or runny nose. After cleaning your nose and waiting 15 minutes anxiously, the result is negative.

While it is a relief to have a negative result, there is also uncertainty. Am I really Covid-free? Or did the test just not detect it? Should I test again? Can I be around other people?

The confusion is justified, say testing and public health experts. It stems from a lack of understanding of how tests work. Rapid antigen self-tests look for pieces of viral protein from a swab stuck in your nose and are designed to identify if you have an infectious level of the virus. But a negative test is not a guarantee that you don’t have Covid.

It could be that your symptoms are an immune response signaling the arrival of Covid or another invader. The more your immune system is working to contain the virus, particularly an immune system that is overloaded with vaccine antibodies, the more likely you are to get an early negative result on a rapid test, even if you are infected.

“It could be that the virus in your body is playing a tug of war with your immune system,” said Michael Mina, director of science at eMed, a company that helps rapid test users get treatment at home.

“If you test negative and you have symptoms, don’t assume you’re negative. Assume the virus hasn’t had a chance to grow yet. The symptoms could mean your immune system is just triggering a very early warning.”

Mina advises people to get a quick test on the first day of symptoms. A positive result means you almost certainly have Covid. If the result is negative and symptoms persist, you should still take precautions, wear a mask, and avoid close contact with others.

If you cannot test daily, wait 48 hours and test again. If you are still negative but your symptoms persist or are getting worse, you should have another test on day 4. Or you can go to a testing center for a PCR, which can sometimes detect Covid a little earlier than a test. at home, although you may have to wait a day or two for results.

Experts say that if you have symptoms and keep getting negative self-test results, it could be that your immune system is doing a good job of beating the virus. Or it could be that you have another illness. In any case, you should try to avoid infecting other people.

“If you have symptoms and you continue to test negative, your chances of being infected with Covid are greatly decreased,” said Robert Wachter, chair of the department of medicine at the University of California, San Francisco. “But you should probably wear a mask that day, because you have something.”

And remember, your self-test result is just information. If you haven’t left the house in weeks, your negative result after a few tests is probably correct. If you have symptoms and have spent time in bars or a family member has been exposed to Covid, you should be extra cautious, even if initial results are negative. It could be that you tested too early and your viral load is not high enough to be detected.

When Jillian Horton, an internal medicine doctor in Winnipeg, started feeling sick, she was pretty sure she had Covid. Her husband had been exposed and also had symptoms. She decided to run an experiment, testing herself several times over the course of a few days to track the dynamics of the virus.

“With my husband testing positive and me very symptomatic, I was pretty sure I had Covid,” Horton said. “I was curious to see what I could identify in terms of when I could turn positive.”

Horton’s husband got sick one Friday night, and that night she tested negative. On Saturday, she started feeling sick and tested herself three times throughout the day. All three results were negative.

On Sunday morning, she woke up feeling worse. At 6am, she tested and saw a faint line on the test — which she called a “weak positive”. She took two more tests on Sunday and both came back negative. On Monday morning, she tested again, and the test quickly came back positive.

What is remarkable about Horton’s experiment is that if she had tested at a different time on Sunday, she might not have discovered the faint positive. Her immune system was clearly fighting the virus, as evidenced by her two negative results at the end of the day.

Horton noted that testing at the right time to pick up a high viral load was similar to putting a net in a stream. If the fish isn’t there, you won’t catch anything. But if you expect fish to be plentiful, you’ll catch your dinner.

Horton said she was concerned that many people think tests aren’t working when in fact they are a useful tool if you understand how to use them. They are ideal for “enacting” Covid, but more information needs to be considered when evaluating a negative test.

“I often hear people say, ‘The test is useless,'” Horton said. “What my experience has illustrated is that when you have symptoms, the tests are really ‘decision’ tests. I think about those two days when I was so symptomatic. I had one positive test and five negative ones. There was only one moment when I was more contagious.”

Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and one of the world’s leading experts on viral transmission, said she assumed her daughter had Covid even after a quick test came back negative. The child had a fever and a sore throat and had been exposed to Covid through his gym team.

But the tests proved useful in knowing that their daughter was not highly contagious, which helped Marr’s family know how to manage the risk. “We knew we needed to be careful,” Marr said.

“But we didn’t have to totally lock her up. The test told us her viral load wasn’t high enough that we had to lock her in her room and worry that we would all catch the disease.” Instead, the family wore masks and opened windows to improve ventilation.

Kristina Kasparian, who works from home in Montreal, believes she may have contracted Covid from her husband, who is a professor. They disagreed on whether a self-test he took showed a weak positive. But a few days later she woke up with a tightness in her chest and a sore throat. Her test was positive, and her husband kept testing negative.

“It’s great to have this tool, but it’s such a time-varying snapshot,” she said.

Mina said that despite the limitations, people would benefit from frequent testing whenever they suspect they’ve been exposed, have symptoms, or want to be sure they’re not infectious before spending time with a high-risk person. He also recommends testing before starting to interact with others again to make sure it remains non-infectious.

“These tools bring enormous benefits during a pandemic like this,” said Mina. “They’ll catch you when you’re most contagious. They’ll even catch you most of the time when you’re just a little infectious. They’ll catch almost everyone when they have a viral load high enough to spread. But they’re not perfect. .”

^{Translated by Luiz Roberto M. Gonçalves}