As covid-19 infections have skyrocketed across much of the United States, largely driven by the highly contagious delta variant, so have demands for access to testing, including at-home test kits that can answer the urgent question, "Do I have the coronavirus" within a matter of minutes.

In recent weeks, Google searches for "home coronavirus test" and related queries have sharply risen, closely mirroring the surge in cases, including some breakthrough infections among fully vaccinated people, that have prompted many Americans to return to wearing masks and following pandemic-related restrictions.

"While there was a bit of a lull in awareness and diligence, now people are seeing these numbers. They're having family members who are being affected," said Matthew Binnicker, the director of Clinical Virology at the Mayo Clinic in Rochester, Minn. "There's kind of this renewed sense of urgency, 'Hey, we better get tested if we think we've got a close contact or if we have any symptoms.' "

Several at-home coronavirus test kits have received emergency-use authorization from the Food and Drug Administration and can be purchased without a prescription online or from drugstores, giving people access to a much more convenient testing option. "You have the result in 15 minutes and you don't need to go anywhere," said Clare Rock, an infectious-disease physician and associate professor of medicine at Johns Hopkins School of Medicine. She added, "People have the autonomy to take some of this into their own hands."

But experts emphasized the tests are not 100 percent accurate and a negative result shouldn't be thought of as a "free pass." Here's what else you need to know about home tests and what the results can - and can't - tell you.

Q: What is the difference between antibody, antigen and PCR tests?

A: Antibody tests look for immune responses to infection or vaccination. Though such a test can tell you if you've had the coronavirus at some point in the past, experts said it's not useful for identifying active infections. "They're really not good or intended for diagnosing someone with covid-19 because it takes 10 to 12 days before what that test is looking for is present," Binnicker said.

If you're trying to screen for a coronavirus infection or figure out if your cough or fever is covid-related, experts recommend using antigen and molecular diagnostics. Both tests are looking for viral components: Antigen tests search for protein pieces of the virus while molecular tests, such as the "gold standard" PCR tests, detect RNA, the virus's genetic material.

Most of the over-the-counter home testing options authorized by the FDA, which include the Abbott BinaxNOW, Ellume Covid-19 Home and Quidel QuickVue tests, are rapid antigen and can typically return results in 10 to 15 minutes. Some tests also use an app or video proctoring to record results. While molecular tests, such as PCR tests, usually can't be done at home, there is at least one authorized rapid at-home molecular test that doesn't require a prescription. The Cue Covid-19 test is expected to be available for direct consumer purchase later this fall, according to a company spokesperson. People can also purchase kits to collect samples at home to be sent in 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. Tests can be purchased online or at your local pharmacy, but experts noted that there's no guarantee they will be available and there may be a limit on how many one person can buy.

"On Amazon, etcetera, some of the tests are basically sold out," Rock said. "There obviously is significant demand for the tests." (Amazon founder Jeff Bezos owns The Washington Post.)

Q: How accurate and reliable are at-home tests?

A: Some tests "work better than others," said Albert Ko, chair of the epidemiology and microbial diseases department at the Yale School of Public Health. Accuracy depends, in part, on test sensitivity and specificity, Ko said. Test sensitivity is how well it can identify individuals infected with the coronavirus. 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 percent 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." But while 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 added, "what this test is probably not good at, especially for the asymptomatic," 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.

Q: What is the cost of at-home tests and are they covered by insurance?

A: Authorized over-the-counter at-home rapid antigen tests can range in cost from about $20 to $35. While that is "relatively cheap" compared with some home collection kits and PCR tests, which can cost $100 or more, "if you're talking about testing a family a couple of times a week, that adds up," 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.

It's unclear whether home tests are universally covered by insurance. A February guidance issued by the federal government strengthened coverage requirements for coronavirus diagnostic testing, but only mentioned point-of-care testing and tests administered at state or local sites.

It's possible, Binnicker said, that if a home test is ordered by a physician, there is a greater chance it may be covered.

Q: How do I use an at-home rapid antigen test?

A: Home test kits come with detailed instructions for obtaining a sample - usually via a nasal swab - and testing it, and it's critical to follow the steps closely. "The test is only as good as the sample is," Ratner said. "If you get a negative result on a suboptimal sample, that might not be a real negative result."

In addition to testing yourself if you have symptoms, the Centers for Disease Control and Prevention recommends testing three to five days following exposure to someone with suspected or confirmed covid. If you test too soon or too long after you've been exposed to covid, the test could produce a false negative, because a person's viral load changes over the course of the infection. If you're asymptomatic or don't know whether you might have been exposed, timing a test can be more challenging, experts said.

Q: What about tests for kids?

A: The Abbott BinaxNOW, Ellume and Quidel QuickVue rapid antigen tests are authorized for at-home use in children as young as 2, but experts said the amount of data on their performance among kids 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," Ratner said.

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, however, is they may tolerate the nasal swab better, Rock said. 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 percent of the school population every week, you probably wouldn't need to test at home.

Q: How should I think about my results?

A: 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 free 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. 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 of time.

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.

That is one of the reasons some manufacturers of home tests kits include at least two tests in each kit along with recommendations to conduct a second test at least 36 hours into the future. Having an extra test also accounts for human error, Binnicker said. In the general population, "you're going to have higher rates and higher incidence of people just kind of messing up on the testing and maybe not doing it correctly the first time," he said.

But "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."

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 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."

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

At-home rapid tests are just one "tool in the toolbox," Rock said. People should still be prioritizing vaccination and other strategies that can help lower the risk of spread. "Each of these strategies reduces risk further. Nothing can reduce risk to zero, but they each play a part."