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現在の単語数: 2366語

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作成日: 2022/08/13 14:28

更新日: 2025/12/09 14:22

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Your Questions About Home COVID-19 Tests, Answered
Catherine Roberts
Are rapid antigen tests accurate? Should you test more than once? With the pandemic ongoing, we have expert answers to these questions and more.

By Catherine Roberts

As the various sub-variants of omicron continue to circulate in the U.S., rapid at-home tests remain an important tool for protecting yourself and others. Home tests are appealing because they allow you to avoid going to a testing center and they produce results in minutes.

But confusion persists about how accurate they are, and how and when they’re most useful. Here, answers to questions about rapid tests, including new guidance from the Food and Drug Administration.

What Kind of COVID-19 Tests Can I Take at Home?
Of the at-home, over-the-counter COVID-19 tests authorized by the FDA, most are rapid antigen tests. These are the ones consumers are probably most familiar with, like BinaxNOW, Flowflex, and QuickVue, that you can pick up at a pharmacy or online, or that you can order for free via the U.S. Postal Service (in limited quantities) at this site. Home tests return results in under an hour.

There are also some options for at-home molecular tests (which are generally more accurate than antigen tests; see below for more information). PCR tests are available—LabCorp’s Pixel is an example—but they require mailing your sample to a lab and waiting for a result. Rapid at-home molecular tests are also available, from companies like Cue and Lucira, but they’re pricey and thus not widely accessible to most people.

How Do Home Antigen Tests Work?
Antigen tests detect proteins on the surface of the SARS-CoV-2 virus that provoke your immune system to respond. Because they react only to proteins actually contained in the test sample, they can return a false negative result when virus levels are low, as is generally the case, for example, when someone is infected but hasn’t yet started showing symptoms.

That’s in contrast to the molecular-based tests, including PCR tests, which detect the genetic material of a virus. These tests work by amplifying any existing genetic material present in a sample, by as much as a billionfold, according to Nathaniel Hafer, PhD, director of operations for the UMass Center for Clinical & Translational Science and an assistant professor in the program in molecular medicine at the UMass Medical School in Worcester, Mass. “This allows these tests to detect extremely small amounts of genetic material in a sample,” he says. “This amplification step makes them highly accurate, whether or not a person has symptoms.”

Are Home Antigen Tests Accurate?
Yes. “At-home antigen tests are quite accurate, especially if you have symptoms,” Hafer says.

Still, they have some limitations. Antigen tests are more likely to return a false negative if you’re infected but you don’t yet have symptoms. That’s because they might not be able to pick up the low viral load of an early or waning COVID-19 infection.

You also have to use them correctly. According to the FDA (which recently issued a warning to remind consumers about this), the antigen tests currently being offered in the U.S. are all meant to be used with what’s called a serial testing method. That means to get the best accuracy, you need to take at least two tests over the course of two or three days if the initial test is negative.

This is particularly important if you don’t have symptoms since, again, the tests aren’t as sensitive to asymptomatic infections. In fact, the FDA advises that if you don’t have symptoms, but you’ve been exposed to COVID-19, that you should take a total of three tests, each 48 hours apart, to be certain you’re not infected. (You can abandon the serial testing strategy once one of the tests turns up positive—a positive on a rapid test is generally very accurate.)

It’s also possible that since antigen tests work by directly detecting virus present in your nose, and higher viral loads mean greater infectivity, they may be a reasonable marker of whether someone is infectious, says Larissa May, MD, a professor of emergency medicine at UC Davis in California, at least at the time they take the test. Still, more research is needed on how well antigen test results can indicate a person’s infectivity, notes the Infectious Diseases Society of America (IDSA), since plenty of other factors are at play that could influence whether you’re likely to infect someone. These include your body’s own level of immunity and the characteristics of a given situation like ventilation and how well people’s masks fit.

The likelihood of a false result on an antigen test also varies depending on the prevalence of COVID-19 in a community, a March 2021 Cochrane review found. When there’s little virus circulating in the area, a positive result on an antigen test is more likely to be a false positive, and you should confirm the result with a PCR test to avoid needlessly isolating yourself. However, when there’s a lot of COVID-19 circulating, as there is in most places in the U.S. right now, the researchers found that there’s a higher risk of a false negative result on an antigen test.

The different rapid antigen tests available have different levels of accuracy. The Medical Letter, a publication for physicians, offers this chart summarizing their accuracy (PDF) based on data from the test manufacturers. (The PPA, or positive percent agreement, and NPA, or negative percent agreement, indicate the rate at which the antigen test results agree with a positive and a negative PCR result.) Keep in mind, however, that some independent studies of test accuracy have found that the real-world accuracy may be lower than what manufacturers report in their data, at least when it comes to the ability of a test to return a true negative result in someone without symptoms.

How Much Do Home Antigen Tests Cost, and Where Can I Get Them?
Drugstores such as CVS, Walgreens, Walmart, and others sell antigen tests over the counter, generally both online and in stores. Prices for a package of two tests start around $20. Some cities and states may still be distributing free at-home antigen tests, so check to see what may be available near you.

People with private insurance can get at-home antigen test kits from pharmacies and retail stores free of charge or at a discount. The federal government is incentivizing health plans and insurers to set up a network of retailers across the country where plan members can get tests with no up-front costs. You’ll need to check with your own health plan for details on where such tests are available.

If you go to a pharmacy or store that’s not in your insurer’s network for this program, you can purchase tests and be reimbursed by your insurance company for up to $12 per test. Be sure to keep your receipt for reimbursement.

And as mentioned above, you can order free tests from the federal government, at COVIDTests.gov.

For more on how to get free tests, see our report here.

How Do I Use a Home Antigen Test?
The exact directions will vary by brand, so make sure to read them carefully before you begin. The Centers for Disease Control and Prevention recommends that you disinfect the surface on which you’ll be placing the components and wash your hands. Lay out all the materials in the test kit before you start the procedure, as if you’re practicing the chef’s technique of “mise en place” but for an infectious disease diagnosis instead of cooking.

The tests involve swabbing the inside of your nose—and it’s crucial to do this part correctly, so that you gather a good sample. Directions generally say to:

• Insert the soft end of the swab no more than ¾ inch into your nose.

• Firmly and slowly swirl the swab in a circle around the inside wall of one nostril at least four or five times, for a total of 15 seconds. Repeat this process using the same swab in the other nostril.

You then put the swab containing your sample into contact with some component of the test, such as into a tube of liquid that you’ve unsealed or against a card containing a test strip. Once you’ve finished all the steps, you’ll need to wait for a result. Be careful about keeping track of the time, according to what the test’s directions indicate. The results won’t be valid until a certain amount of time, but they’ll also become invalid if you wait too long. CareStart and QuickVue’s tests, for example, should be read 10 minutes after completing the procedure, but they become invalid after 15 minutes.

Should I Swab my Throat, Too?
Some anecdotal advice has circulated that if you suspect you have a case of COVID-19 but get a negative result on an at-home antigen test, you should try the test again but this time swab your throat as well as the inside of your nose. The theory behind the advice goes like this: Some preliminary evidence suggests that the omicron variant begins replicating fastest in the throat, rather than the nose as earlier variants did. That means you might have better luck getting a true positive result on a test earlier in the course of an infection by swabbing your throat rather than just your nose, because a nose swab might return a false negative result for longer.

This practice would make sense—if we had the evidence to back it up, says Stephen Master, MD, president of the American Association for Clinical Chemistry and the chief of the division of laboratory medicine at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia. But the antigen tests currently available in the U.S. were designed and authorized to use nasal swabbing. There aren’t a lot of studies that look at how well throat swabbing works with these tests. And different biological fluids—saliva vs. the mucus in your nose, for example—can have different effects on the result of the test.

In one early study involving 49 people who were confirmed to have omicron infections, BinaxNOW tests performed with nasal swabs correctly detected 86 percent of the cases [?], but just 47 percent of infections performed with throat swabs. The researchers concluded that throat swabs shouldn’t replace nasal swabs with rapid tests. As for swabbing both areas, the study found that test accuracy went up by just 3 percentage points to 89 percent.

But that’s just one study. For now at least, the FDA advises against using a throat swab with the rapid tests authorized in the U.S.

Still, in the meantime, if you do go ahead and try out the throat swab, use care. In the U.K., where tests that combine throat and nose swabs are already common, Public Health England advises that you swab the throat first, and then the nose. Here’s a video with step-by-step instructions.

If your nose-swab-only test comes back negative but your throat-swab test comes back positive, what should you do? “I would view that in the same way as I would view someone having symptoms and not being positive yet by a nasal swab,” Master says. When case prevalence is high, it’s wise to behave as if you’re likely infected, at least until you can double check with follow-up testing.

What Should I Do After a Positive or Negative Result?
If you test positive on an antigen test, you can trust it even if you don’t have symptoms, so you should take the recommended isolation measures.

If you get a negative result on an antigen test but you have symptoms or you think you were exposed to COVID-19, follow up with a second antigen test 48 hours later. If the second test is negative, what you should do depends on your situation, according to the FDA. If you have symptoms, you can either try a rapid test a third time (again, 48 hours later) or seek out a PCR test (or call and consult a doctor). If you don’t have symptoms but were recently exposed to the virus, test a third time after another 48 hours.

The bottom line: a single negative antigen test doesn’t necessarily mean that you aren’t carrying the virus and that you wouldn’t test positive if you repeated the test the next day.

“It doesn’t sort of give you a free pass to assume that there’s no way you could spread COVID,” Susan Butler-Wu, PhD, an associate professor of clinical pathology at the University of Southern California in Los Angeles, told the American Association for the Advancement of Science’s SciLine in an interview earlier this year. This helps to explain how people can contract COVID-19 at events such as concerts that require negative rapid tests right before entry.

Still, combined with other mitigation measures, such as masking and ventilation, getting a negative result on a rapid antigen test right before joining an event like a family gathering can help provide another layer of protection to attendees. You’ll want to take the test as close to the start of the event as possible—such as at the door. May says negative rapid antigen test results can also provide some added assurances in school settings where, again, other measures like masking and ventilation are also being used.

How Should Home Tests Be Stored?
Keeping tests at room temperature is fine. A study in the Journal of Clinical Virology found that storing kits above 86° F or below about 37° F decreased the accuracy of the tests, but most home environments are likely to be more temperate than either of those extremes. Test kits do have an expiration date, but in some cases the date might have been extended by the FDA after the kits were sold. To find out whether the expiration date on your test has been extended, visit this FDA site and find the brand of test you’re using on the list.

—Additional reporting by Lauren F. Friedman

Consumer Reports is an independent, nonprofit organization that works side by side with consumers to create a fairer, safer, and healthier world. CR does not endorse products or services, and does not accept advertising. Copyright © 2022, Consumer Reports, Inc.

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