As millions of Americans scramble to navigate the holidays amid rising coronavirus cases and the rapid spread of the omicron variant, convenient testing options that can quickly detect an infection are becoming harder to find.

“Getting your hands on a test prior to Christmas at this point is probably going to be very difficult because of the demand,” said Matthew J. Binnicker, the director of clinical virology at the Mayo Clinic in Rochester, Minn., speaking about rapid home test kits.

Public health officials are urging Americans to get tested before traveling or gathering with friends and family members, citing rapid tests as an important tool for curbing a variant that is expected to cause record-high COVID-19 hospitalizations in the United States. But the high demand for tests has left major retailers such as Walmart, Walgreens and CVS struggling to keep kits stocked in stores, and online orders have also been affected.

While there may still be time before Christmas for people to get results from lab-based PCR tests, depending on where they are in the country, Binnicker said the outlook for being tested before New Year’s celebrations may be better. “There’s still more time for pharmacies and stores to be able to get access to those kits,” he said.

Here’s what else you need to know about home tests — how they work, where to get them and when to take them. The information and recommendations in this FAQ are drawn from the Food and Drug Administration, previously published Washington Post reports and new interviews with experts.

Please keep in mind that as the pandemic evolves, information about testing options is likely to change.

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What kind of tests are in at-home kits?

The FDA has given emergency use authorization to nearly a dozen over-the-counter, self-collected rapid antigen testing kits, such as the Abbott BinaxNow and Quidel QuickVue tests. These tests look for protein pieces of the virus to detect infection and can provide results in 10 to 15 minutes.

Polymerase chain reaction, or PCR, tests — a type of molecular coronavirus test — also look for components of the virus, in this case RNA. But, unlike rapid antigen tests, most require a laboratory setting and can take up to several days to return a result. (Antigen and PCR tests differ from antibody tests, which look for immune responses to infection or vaccination. Those tests can tell you whether you’ve had the coronavirus at some point, but experts say they are not useful for identifying active infections.) PCR tests are, however, widely considered more reliable than antigen tests.

While molecular tests usually can’t be done at home, there are at least two authorized rapid at-home molecular tests that don’t require a prescription. The Cue coronavirus test and the Lucira test kit are available for direct consumer purchase. People can also purchase kits to collect samples at home to be sent to a lab for analysis.

Whatever test you use, make sure it has received authorization by checking the packaging or product description, or referring to the FDA’s online database.

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Does an at-home test detect omicron?

Research into whether the tests can detect the latest variant, omicron, has just started. But in a recent update, the FDA announced that preliminary lab studies suggest that two brands, the Abbott BinaxNow and Quidel QuickVue antigen tests, “are able to detect the omicron variant with similar performance as with other variants.”

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The FDA noted that those findings came from using heat-inactivated samples of the variant and that necessary research using live virus is ongoing. It will also be evaluating tests from other brands and posting updates to this webpage.

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Where can I find a test?

Tests kits (which usually contain two tests) can be purchased online or at your local pharmacy, but there’s no guarantee of availability, and several major retailers have placed limits on how many kits one person can buy.

Many home tests being sold by Walmart, Walgreens and CVS are listed as “out of stock” online or available only for in-store purchase. Walgreens has instituted a four-item limit on kits purchased in stores or online and CVS is limiting buyers to six kits, USA Today reported. Amazon shoppers can purchase a maximum of 10 kits, but many product listings say orders won’t arrive until after the holidays.

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How much do at-home tests cost, and can I get them free?

Over-the-counter rapid antigen tests can generally cost anywhere from about $10 to $30. President Joe Biden has pledged to have insurance companies reimburse customers who purchase at-home tests, but that plan won’t begin until January.

The president announced Dec. 21 that a half-billion free rapid tests will start being delivered to homes in January and that health officials will set up a website where Americans can order them. The federal government is also establishing testing sites across the country, starting with one in New York City this week.

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Meanwhile, the Centers for Disease Control and Prevention has started to distribute free test kits at a handful of airports, with plans to expand, and some local governments offer free supplies as well. Some school districts are sending students and staffers home for the holidays with tests.

D.C. has been distributing at-home collection kits and offering free PCR testing at eight firehouse sites and other community centers. Beginning Dec. 22, D.C. residents can get a maximum of two rapid tests at eight libraries in D.C., six days a week, and the city is expanding the number of sites where at-home PCR tests can be picked up and dropped off. After the holidays, D.C. Public Schools will cancel classes Jan. 3 and 4 so families and staffers can pick up tests at schools and use them before returning to in-person learning on Jan. 5.

In New York City, there are plans to distribute half a million rapid self-testing kits to community groups. Officials also recently announced that additional testing sites would be opened, bringing the total number of city-run sites to more than 100, The New York Times reported.

Critics say the United States has not invested enough in testing and could have been far better prepared for the supply scramble as the omicron variant spreads. Costs will keep people from testing as often as they need, they say, while the reimbursement process may be too cumbersome and will start too late for a holiday surge of disease.

Carlos del Rio, a professor of medicine and infectious disease at the Emory University School of Medicine, said rapid testing should be just as accessible to Americans as coronavirus vaccines. “They should be subsidized so you can go into the drugstore and buy them for, I mean, no more than $5,” he said.

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What should I do if I can’t find an at-home test?

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Experts still recommend trying to get tested. You should prioritize getting a PCR test if you’re showing any symptoms, said Albert Ko, chair of the department of epidemiology and microbial diseases the Yale School of Public Health.

For people who are using tests to help lower the risk of gathering, try scheduling an in-person test, but remember that PCR tests typically need to be done 24 to 48 hours in advance of an event to make sure you can get results back in time, Binnicker said. Some testing sites may also be closed on or around the holidays.

This close to Christmas, people who haven’t been able to get a test yet might be out of luck, Binnicker said. But he suggested calling different testing sites and asking how long it typically takes for results to come back and whether the site is doing rapid PCR testing.

In the absence of testing, “we have to rely on what we’ve done previously in the pandemic,” Ko said: Get vaccinated and boosted, if eligible. Exercise extra caution around people who are at increased risk of severe COVID-19, and consider not going to an event if a high-risk person is also attending. Additionally, try to stay masked as much as possible (some experts are recommending upgrading masks to N95s, KN95s or double-masking for better protection), keep the gathering size small and consider the risks of any unessential travel., keep the gathering size small and consider the risks of any unessential travel.

But not being able to get a test also doesn’t necessarily mean your holiday plans are ruined, Binnicker said.

“For those who just can’t find an at-home test or who can’t get a PCR test back in time, if it’s a relatively small gathering with less than 10 people and everyone’s vaccinated and everyone wears a mask, it creates an environment where it’s safe to get together,” he said, “especially if there’s no high-risk individuals present.”

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When should I test myself?

Rapid self-testing can help infected people isolate earlier and quickly notify people they’ve spent time with. But if you test too soon or too long after you’ve been exposed to someone with the coronavirus, the test could produce a false negative, because a person’s viral load changes over the course of the infection.

Del Rio advises people who learn of a coronavirus exposure to self-test right away: “You could have already been infected because you’ve been around the same network of people,” he said. If the test comes back negative, he said, you’re not in the clear — you should quarantine and test again five or six days after your exposure.

If you still get a negative result, del Rio said, you are probably not infected.

Experts also advise that testing before an indoor get-together is “especially important before gathering with unvaccinated children, older individuals, those who are immunocompromised, or individuals at risk of severe disease,” the CDC says.

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Should I take more than one test?

Tests typically come in two-packs for a reason, experts said: Testing multiple times increases the chances you will detect an infection, and it helps account for human error. One study found that rapid antigen tests identified at least 98% of coronavirus cases when they were used every three days in a 14-day span.

“Frequent rapid diagnostic testing is critical for restricting community spread,” the researchers said. The downside: Supply shortages, cost and other barriers have made such testing “difficult to achieve.”

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Del Rio recommends testing a day before an event and again shortly before it. When he recently had a small group of people over, he said, “everybody got tested in their car before they came to the house.”

Home test kits come with detailed instructions for obtaining a sample — usually with a nasal swab — and testing it, and it’s crucial to follow the steps closely.

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How accurate are at-home tests?

Some tests “work better than others,” Ko said. Accuracy depends, in part, on test sensitivity and specificity. Test sensitivity is how well it can identify coronavirus infections. Test specificity refers to the likelihood of getting a false positive result.

In general, antigen tests are less sensitive than molecular tests. A scientific review published by the Cochrane Library found that in people with confirmed COVID-19, antigen tests correctly identified infections in an average of 72% of people with symptoms, compared with 58% of infected people without symptoms. (The review included tests that aren’t currently FDA-authorized.)

Antigen tests, Ko said, tend to perform better when people are symptomatic and “spewing out a lot of virus.” At-home rapid antigen tests have pretty high specificity and are probably fairly reliable in confirming that you have the coronavirus and are infectious, he said.

But “what this test is probably not good at, especially for the asymptomatic,” Ko added, is accurately ruling out an infection. In other words, at-home antigen tests are more likely to give you a false negative, particularly if you’re symptomless, than a false positive.

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To increase confidence in a negative test, experts recommend testing frequently at regular intervals, also known as “serial testing.” For example, consider testing more than once in the days leading up to an important event where you might be gathering with vulnerable people, such as grandparents, or continue to test throughout a visit that spans multiple days.

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What should I do after getting my test result?

Testing negative can provide some reassurance that you’re not infectious at the point that you take the test, but it doesn’t give people a “pass to go and do whatever they want to do,” Binnicker said. A single negative result “basically tells you, ‘I’m probably not shedding really high amounts of SARS-CoV-2 right now,’ ” he said, using the technical name for the novel coronavirus.

But that might not be the case in as little as six or eight hours, because the amount of virus in a person’s respiratory tract can change in a relatively short period.

“If you really have to do something where you think it’s high-risk to others and you want to make sure that you’re absolutely not infected,” Ko said, “I would get the PCR test to be sure.” (It’s important to keep in mind that PCR tests also are not 100% accurate.)

If your rapid antigen test comes back positive, you should contact your physician to report the result and consider scheduling a lab-based test, Binnicker said, noting that there is a small chance that the result is a false positive.

“There are important decisions like quarantine, isolation, staying home from school and work that can come from a positive test,” he said. “Confirming that result with a lab-based test is a really good idea.”

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If the test result is in any way surprising to you — say you have symptoms and a known exposure, but you test negative — that’s a situation where you should get a more accurate test.

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What about tests for children?

Some rapid antigen tests are authorized for at-home use in children as young as 2, but experts said the data on their performance among children is limited. “There’s this assumption that the tests perform comparably in adults and children, but I don’t know that that’s a safe assumption,” said Adam Ratner, a member of the Committee on Infectious Diseases for the American Academy of Pediatrics and director of the pediatric infectious-diseases division at NYU Langone Health.

The FDA authorization specifies that if the kits are used for children, an adult must obtain the sample and process the test. Accuracy may be compromised because the adult needs to “know what they’re doing, be able to hold down potentially a squirmy kid and get a good sample,” Ratner said, “and it’s not easy.”

One benefit of using home tests for children is they may tolerate the nasal swab better, said Clare Rock, an infectious-disease physician and associate professor of medicine at the Johns Hopkins School of Medicine. In home tests, the swab typically doesn’t need to be inserted as deeply as in lab-based tests.

When deciding whether to start testing your children at home, you should consider factors such as the rate of infections in your area and the risk mitigation strategies enforced at your child’s school or day care, Ratner said. If, for instance, your child is going to a school that has implemented universal masking, social distancing, improved ventilation and random testing of 10% to 20% of the school population every week, you probably won’t need to test at home.

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