Publisher Health

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

That’s the question that has confounded many people who have reached for a home test because they have a sore throat, cough or runny nose. After swabbing their nose and waiting an anxious 15 minutes, the result is negative.

While there’s relief in getting a negative result, there’s also uncertainty. Am I really free of Covid? Or did the test just not detect it? Should I test again? Can I spend time with other people?

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

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

“It may be that the virus in your body is having a tug of war with your immune system,” said Dr. Michael Mina, chief science officer for eMed, a company that helps rapid test users get treatment from home. “If you test negative and you have symptoms, don’t assume you’re negative. Assume that the virus has not had an opportunity to grow up yet. The symptoms might mean your immune system is just triggering a very early warning.”

Dr. Mina advises people to take a rapid test on the first day of symptoms. A positive result means you almost certainly have Covid. If the result is negative and your symptoms continue, you should still take precautions, wear a mask and avoid close contact with other people. If you can’t test daily, then wait 48 hours and test again. If you’re still negative but your symptoms persist or are getting worse, you should take another test on Day 4. Or you may want to go to a testing center to take a P.C.R. test, which can sometimes detect Covid a little sooner than a home test, although you may have to wait a day or two for the results.

Experts say that if you have symptoms and continue to get negative results on home tests, it may be that your immune system is doing a good job beating the virus. Or it could be that you have another illness. Either way, you should try to avoid infecting others.

“If you have symptoms and continue to test negative, the chances that you’re infectious with Covid have gone down a lot,” said Dr. Robert Wachter, the chair of the medicine department at the University of California, San Francisco. “But you probably should wear a mask that day because you have something.”

And, remember, the result of your home test is just one piece of information. If you haven’t left the house in weeks, your negative result after a few tests is probably accurate. If you have symptoms and you’ve been spending time in bars or a family member has been exposed to Covid, you should be more cautious, even if the initial results are negative. It may be that you tested too early and that your viral load isn’t high enough to be detected.

When Dr. Jillian Horton, an internal medicine doctor in Winnipeg, started feeling ill, she was pretty sure she had Covid. Her husband had been exposed and had symptoms, too. She decided to conduct an experiment of one, testing herself several times over the course of a few days to track the dynamics of the virus. “With my husband testing positive and myself very symptomatic, I was sure I had Covid,” Dr. Horton said. “I was curious to see what I could pinpoint in terms of when I might flip positive.”

Dr. Horton’s husband became ill on a Friday night, and that evening she tested negative. On Saturday, she began to feel sick and tested herself three times throughout the day. All three results were negative.

By Sunday morning, she woke up and was feeling worse. At 6 a.m. she tested and saw a faint line on the test — what she called a “weak positive.” She took two more tests on Sunday and both were negative.

On Monday morning, she tested again, and the test rapidly turned positive.

What is notable about Dr. Horton’s experiment is that if she had tested at a different time on Sunday, she may never have discovered the weak positive. Her immune system was clearly battling the virus, as evidenced by her two negative test results later in the day.

Dr. Horton noted that testing at the right time to catch a high viral load was similar to putting a net in a stream. If the fish aren’t there, you won’t catch anything. But if you time it so that the fish are plentiful, you’ll catch your dinner.

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

“So often I hear people say, ‘The test is useless,’” Dr. Horton said. “What my experience illustrated is that when you have symptoms, the tests are really ‘rule-in’ tests. I think of those two days when I was so symptomatic. I had one positive test and five negative tests. There was only one moment in there where I was more infectious.”

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 rapid test came back negative. The child had a fever and sore throat, and she had been exposed to Covid through her gymnastics team.

But testing proved useful for knowing that her daughter wasn’t highly contagious, which helped Dr. Marr’s family know how to manage the risk. “We knew we needed to be careful,” Dr. Marr said. “But we didn’t have to totally put her in jail. The test told us that the viral load was not high enough that we had to lock her in her room and be that worried about all of us getting it.” Instead, the family wore masks and opened windows to improve ventilation.

Kristina Kasparian, who works from home in Montreal, believes she may have gotten Covid from her husband, who is a schoolteacher. They disagreed on whether a home test he took showed a faint positive. But a few days later she woke up with tightness in her chest and a sore throat. Her test was positive, and her husband has continued to test negative.

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

Dr. Mina said that despite the limitations, people would benefit from frequent testing any time they suspect they have been exposed, have symptoms or want to be sure they are not infectious before spending time with a person at high risk. He also recommends testing after you recover to be sure you’re not still infectious when you start interacting with others.

“These are tools that have massive benefit during a pandemic like this,” Dr. Mina said. “They will catch you when you’re most infectious. They will even catch you most of the time when you’re just slightly infectious. They will catch almost everyone when they have a high enough viral load to spread. But it won’t be perfect.”