If you had COVID-19 symptoms in 2020, you probably would have masked up and braved a visit to a laboratory, doctor’s office, or clinic to get a polymerase chain reaction (PCR) test. A health care worker would have shoved a swab up your nose, and then you would have waited a day or two, if not more, to get your results.

Now, you’d likely use an at-home test, which spits out results in just 15 minutes. It’s easier, faster, and reliable, even against the latest variants. Though they’re not perfect, at-home tests will detect infections about 80% of the time, according to the U.S. Food and Drug Administration.

So is there any reason to get a PCR test anymore?

Judging by the steep drop-off in PCR testing rates, many people seem to have decided there is not. Roughly a year ago, more than 15 million laboratory test results were reported to the U.S. Centers for Disease Control and Prevention each week. Now, that number has dropped to about 2 million per week—and it’s not just because there’s less virus circulating. Studies show that use of at-home tests has steadily increased over time, suggesting that many people are opting to test themselves instead of getting PCR tests.

If you’re feeling fine and simply doing your due diligence before traveling, socializing, or going to an appointment, an at-home test is a great option, says Dr. Charles Chiu, a professor of laboratory medicine at the University of California, San Francisco. If you’re experiencing symptoms and want to know whether they’re caused by COVID-19, rapid tests can also do a good job.

But there are still times when a PCR test is best, Chiu says.

PCR tests are considered the gold standard of COVID-19 detection because they’re highly sensitive, able to detect even small amounts of the virus’ genetic material. The SARS-CoV-2 virus takes some time to build up in the body after exposure, with viral load typically peaking around the time symptoms emerge. A PCR test may detect an infection even before that peak, says Dr. Chaz Langelier, an infectious disease physician with the Infectious Diseases Society of America.

Read More: Stock Up on Free At-Home COVID-19 Tests While You Can

Most at-home tests, by contrast, are antigen tests that detect viral proteins. They’re good indicators of whether someone is currently contagious, but they’re not always sensitive enough to pick up on small amounts of virus, Langelier says. That’s why most antigen tests are meant to be used repeatedly: if someone tests shortly after an exposure, they might not yet have enough virus in their system for the test to detect it. A day or two later, however, they may test positive.

Antigen tests work well if you can do that sort of serial testing, already have symptoms (and thus probably have quite a bit of virus in your system), or want to know about your current contagiousness, such as before an event.

But PCR tests are still the best option for situations where “you only have a single opportunity to test and if you missed an infection that was impending, the consequences could be disastrous,” Langelier says. For example, it’s wise to test people before they’re admitted to a hospital or nursing home, where they could spread the virus to lots of vulnerable people if an antigen test missed a brewing illness, he says.

Anyone experiencing severe symptoms or complications of COVID-19 should also get a PCR test, even if they’ve already tested positive on a rapid test, so their doctors can make a concrete diagnosis and decide on treatment plans, Chiu adds. While a PCR test result isn’t required to get the antiviral Paxlovid under federal guidelines—proof of an at-home test often suffices—some clinicians may order a PCR test to confirm the results before prescribing it. Having a laboratory-confirmed test result may also make it easier to get treatment later on if someone develops post-COVID complications, such as Long COVID.

PCR tests can also be helpful if rapid test results are confusing or seem inaccurate, says Dr. David Andrews, an associate professor of pathology and laboratory medicine at the University of Miami Miller School of Medicine and vice chief of pathology at Florida’s Jackson Health System. If someone has telltale COVID-19 symptoms but keeps testing negative on antigen tests, they may want to get a PCR test to be absolutely sure they don’t have the disease. Some PCR tests can also look for multiple viruses—including SARS-CoV-2, RSV, and influenza—at once, making it easier to zero in on the cause of symptoms, he notes.

Some people also want “diagnostic certainty” for their peace of mind, and PCR tests are still the best option for that, says Dr. Yuka Manabe, a professor and infectious disease specialist at Johns Hopkins Medicine. People who are very concerned about not spreading the virus, particularly to immunocompromised or otherwise vulnerable people, may also want to get PCR tests because they are more reliable, she says.

But Manabe says she’s more concerned with the overall availability of tests, rather than the type of tests people are using. The pandemic’s public health emergency designation is set to end in May, potentially drawing to a close some government-funded programs that provide free COVID-19 resources to people across the U.S. It’s unclear what that will mean for the cost, availability, and use of tests, both PCR and antigen.

“What kind of testing is perhaps less important than having access,” Manabe says. “People should have the opportunity to know their status.”

Write to Jamie Ducharme at [email protected].

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