We’ve all heard the anecdotes: Your friend’s spouse or child gets Covid-19 — a known exposure to the virus, all the hallmark symptoms, a positive test, no question about it. Then your friend starts to feel crummy, so they do a quick swab, but their test result is negative. What gives?

Experts say that rapid home tests are still a helpful tool for stopping the spread of Covid-19, but they’re not foolproof. Here are a few explanations for why you might get a false negative result — and how to increase your chances of accuracy next time.

You’re testing too early

The most likely reason a rapid test would produce a false negative is that there isn’t enough virus circulating in your body. The tests require a lot of virus to be present to turn positive, much more than PCR tests do. When people test early — right after an exposure, or with the first tickle in the throat — viral load tends to be low.

“The tests just aren’t very sensitive,” said Dr. Sheldon Campbell, a professor of laboratory medicine at Yale School of Medicine. “It’s an inherent limitation of this kind of test.”

It can be confusing to have recognizable symptoms and a negative test, but experts say the early signs of Covid-19 — like fever and fatigue — are typically caused by your immune system’s initial response to the virus and are not necessarily a reflection of viral load.

Rapid tests are best used as an indicator of when you’re contagious with Covid-19 rather than when you’re infected with it, said Dr. Paul Drain, an associate professor of global health at the University of Washington. And in order to prevent the spread of the virus, contagiousness is what really matters.

Supporting this idea, scientists in Dr. Drain’s lab found that samples taken from people with Covid-19 who had very low levels of the virus (below what a rapid test can detect) were unable to infect cells in a petri dish. This suggests that people with small amounts of the virus also wouldn’t be able to infect another person, Dr. Drain said.

You need to test again

To receive an emergency use authorization from the Food and Drug Administration, manufacturers must submit data showing that their rapid test is at least 80 percent accurate, which means it will return a false negative one out of five times. Because people often test too early, false negatives are even more common in the real world.

A large meta-analysis of over 150 independent studies of rapid tests reported that, on average, the tests correctly detect a Covid-19 infection 73 percent of the time when a person is symptomatic. For asymptomatic infections, the accuracy drops to 55 percent.

Other research suggests that the accuracy of rapid tests improves a few days into an infection. A large study released as a preprint paper last year showed that rapid tests were only 60 percent accurate on the first day of a person’s infection if they had symptoms. If the person was asymptomatic, the accuracy dropped to just 12 percent. However, doing a second test 48 hours later improved rapid test accuracy to 92 percent for people with symptoms and 51 percent for asymptomatic infections. A third test after another 48 hours improved accuracy to 75 percent for people without symptoms.

Because of this, the FDA now recommends so-called serial testing: If you think you’ve been infected with the coronavirus but test negative, test again in 48 hours, after the virus has had more time to replicate. If you’re still negative, take one more test in another two days. (Unfortunately, for the sake of accuracy, this will mean spending more money on tests.) The FDA made this announcement in November 2022, and manufacturers are required to change rapid test packaging to reflect the new guidelines.

“A negative does not necessarily rule you out of having the disease, and that’s why multiple tests are recommended,” said Nathaniel Hafer, an assistant professor of molecular medicine at the University of Massachusetts Chan Medical School who worked on the repeat-testing study. “If you’re positive, you can feel pretty confident that you’re positive. If you’re negative, that repeat test is really important for increasing the accuracy.”

You’re testing incorrectly

Another possible explanation for a false negative result is user error. To make sure you’re testing correctly, read through the instructions first, even if you think you know what you’re doing.

“A lot of us this far into the pandemic have done tests multiple times, and it’s easy to be like, ‘Yeah, yeah, I know what to do,’” Dr. Hafer said. “Pulling out those instructions and really making sure you go through it step by step is the best thing to do because each test is a little bit different.” For example, tests may differ in terms of how long you need to swab each nostril, how far up your nose you should go, how many drops to use on the test strip and how long you need to wait for a result.

Another tip is to blow your nose beforehand. Tests for Covid-19 detect the virus in the cells that line the inside of your nose, not in your mucus, so you don’t want your snot getting in the way of the sample. Also, make sure you’re going far enough up inside your nose.

“The most important thing you can do to Excellerate the accuracy of the tests is get a good sample,” Dr. Campbell said. “If you’re swabbing up there where it burns just a little bit, that tells you you’re doing it right.”

For the most part, though, people seem to be pretty good at self-testing. One of the few studies that compared test accuracy between health care professionals and people swabbing themselves found no difference in rapid test results. Another study found that children as young as four were able to swab themselves proficiently, with test results matching a health care worker’s swabs 98 percent of the time.

The good news: The tests aren’t losing accuracy over time

Accuracy doesn’t appear to be changing with each new variant. Several studies have found that rapid tests performed just as well on the first Omicron variant as they did on earlier strains of the virus. And although there isn’t data yet, the experts say there’s no reason to think that more recent subvariants like BA.5 and XBB.1.5 are any different. That’s because most of the mutations occur in the spike protein, which the virus uses to enter and infect a cell. Rapid tests detect a different kind of protein, called a nucleoprotein, that has undergone many fewer changes.

“Because the tests are designed to detect nucleoprotein, and because nucleoprotein hasn’t been mutating, we can be pretty confident that the tests are going to still be able to perform as well as they have in the past,” Dr. Hafer said.

If you think you have Covid-19 but test negative, Dr. Hafer recommended waiting 48 hours and testing again. However, if you are over the age of 50 or have a pre-existing condition, Dr. Drain advised getting a PCR test as soon as possible so you can begin taking the antiviral drug Paxlovid if you are positive. Paxlovid can reduce the risk of severe illness, but the medication needs to be started within the first five days of an infection. “The earlier one starts Paxlovid, the more beneficial it would be,” Dr. Drain said.

If you feel awful but you’ve tested negative on three rapid tests in five days (or if you have a negative PCR test), you could be infected with another virus, like influenza or R.S.V. Regardless of what’s making you sick, it’s important that you stay home until you feel better so that you don’t infect other people.