How a COVID Home Test Works and When to Use One

“The way that I would recommend using rapid antigen tests is really as a screening method,” Stephen Kissler, a research fellow in the Department of Immunology and Infectious Diseases at the Harvard T.H. Chan School of Public Health, told reporters on Dec. 7.

It can take days to get results back from the standard PCR tests (short for polymerase chain reaction) you get at many doctor's offices and testing sites. “And by the time you get the test back, oftentimes the result that it gives you is no longer very meaningful,” Kissler says, because it’s possible you got infected during the waiting window.

A rapid test done from the convenience of home, however, provides “immediate, actionable results,” says Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security and an associate professor at the Johns Hopkins Bloomberg School of Public Health. “If you test positive, don't be around other people, because you've got infectious virus in your nose. So it's a good public health tool for making sure that people who are potentially contagious stay to themselves.”

The key is taking the test as close to your plans as possible — aim for the same day, explained Matthew Binnicker, director of clinical virology at the Mayo Clinic, in a recent briefing. “That's going to give you the best information [on] whether someone has high amounts of the virus in their system at that time.”

Which home test is best?

The Food and Drug Administration (FDA) has authorized several over-the-counter home testing kits, and a handful are already available at pharmacies and major retailers, although surging demand may make them difficult to track down in some areas.

That’s why the best test is really “the one you can find on the shelves at your local store,” Kissler says, adding that most of the tests “have pretty comparable sensitivity and specificity for detecting SARS-CoV-2,” which is the official name of the virus that causes COVID-19.

If you are purchasing your test online from an unfamiliar retailer, be sure to confirm that the product has emergency use authorization from the FDA, since the agency has seen fraudulent test kits being sold on the internet. This should be clearly marked on the box. You can also double-check the FDA’s website for a list of authorized COVID tests.

Another tip: Check the label for how long the results take; that could make or break your decision to go with one brand over another.

“I tend to like the tests that give me the results sooner, because oftentimes if I'm using a rapid test, it's because I'm on my way somewhere and I want to know whether or not I've got the virus,” Kissler says. “So a test that gives me a return in 10 or 15 minutes is a little bit more convenient for me than a test that takes an hour.”

Ease of use is another factor to consider when selecting a test. Some kits require weaving a long swab through a card-shaped reader; others call for dunking a test strip in a vial of solution. “It's sort of a matter of personal preference,” Kissler notes.

How accurate are COVID home test results?

It’s true that PCR tests are more sensitive than antigen tests, meaning you’re less likely to receive a false negative if you go that route. But antigen tests “are very accurate when you are most infectious,” Gronvall says. “They're actually equivalent to PCR for that time period when you are most dangerous to others.” A recent study led by researchers at the Johns Hopkins University School of Medicine found that the coronavirus was accurately detected by rapid antigen tests in 87 percent of hospital patients with symptomatic COVID-19 and 71 percent with asymptomatic cases of the illness. When it comes to omicron, the FDA issued a statement in late December that said antigen tests do detect the new variant, “but may have reduced sensitivity.”

False positives are rare with rapid antigen tests, Gronvall adds. So if you test positive with an at-home test, you likely have COVID and should isolate yourself from others. You can always confirm the diagnosis with a follow-up PCR test or another antigen test, since "the accuracy of your result goes up with multiple tests,” Gronvall explains.

If the results are negative, it means the test didn’t detect the virus, but it doesn’t rule out infection completely, the CDC says. It could be that your infection is in the early stages and you don’t have enough virus in your sample for the test to turn positive. Repeating the test at least 24 hours later will paint a clearer picture. Some home kits come with two tests for this very reason.

“Of course, [home tests] don't bring the risk of bringing infection to a gathering down to zero, but they do reduce the odds by an awful lot,” Kissler says. And remember: A negative test is just a snapshot of your status at that moment in time. “[It] really doesn't clear you for very long — basically just that day and maybe the next day,” he adds.

What else should I keep in mind?

Testing, whether it’s done at home or in a health clinic, isn’t a substitute for other prevention measures, like vaccines, masks and physical distance. It’s just another risk-reduction strategy that can help keep people, especially high-risk individuals, safe while the virus continues to circulate at high levels.

Kissler is hopeful there will be at-home testing options for respiratory illnesses beyond COVID, such as flu. They could go a “long way” toward keeping schools and workplaces running smoothly, he says. But in the meantime, all eyes are on the ability for fast and convenient testing to curb winter surges by stopping the spread of COVID, especially as people get together in person.

“We know that super-spreader events, where a single infection infects many others, really drives the spread of this virus hugely. And rapid antigen tests can really reduce the odds of something like that happening,” Kissler says.