FROMArlos del Rio is not slacking off when it comes to the health of his 87-year-old mother. Even when he’s not feeling ill, the day before he plans to visit her, an infectious disease professor at Emory University in Atlanta does a home test for COVID-19. The next day, he takes the test again as soon as he enters her house. “I want to minimize the risk of infection before I see her,” del Rio says.

You don’t have to be an infectious disease specialist to know that an 87-year-old man is at high risk, but double testing, as del Rio does, is not part of any formal protocol on how to interact with a person with such advanced levels. years is just a practice he developed on his own. In this sense, an Expert Advisor is a lot like the rest of us – it follows a book of testing rules that hasn’t really been written yet.

Previously during the pandemic, home tests either did not exist, or when they were allowed, they were difficult to obtain as demand skyrocketed and supply lagged behind. Now there is a lot to have: the federal government will deliver some rapid tests to your home for free, and insurers are required to reimburse policyholders for 8 tests per month so that people can stock up on them to use as needed. But when exactly are they needed? Should I check myself before the trip? After? When do you visit a home with small unvaccinated children? Before or after you go to the dinner party? At the moment, there is no consensus on when to use them.

What then do professionals – scientists specializing in infectious diseases? To find out, TIME interviewed several experts to determine how often they test at home and when they might ask other people to take a test before visiting or interacting with them.

When they have symptoms

If there is any situation in which self-examination is mandatory, it is when you or a family member is experiencing symptoms consistent with COVID-19. But while taking the test is an important first step, the results may not tell you the whole story, warns Thomas Breeze, an assistant professor of epidemiology at Columbia University’s Mailman School of Public Health. Symptoms may appear before a person’s viral load is high enough to be detected with a home test.

“There is a debate about how sensitive these home tests are compared to laboratory tests,” Brize says. “After a negative test, I usually do a second test in a day or two.” A PCR test can also be used as an alternative to a second at-home test, he says, and that’s Brize’s personal preference because PCR tests are more sensitive than at-home tests and are more likely to give a more accurate result. The downside, of course, is that the PCR test requires a visit to a clinic or testing center, and results take longer—usually 24 hours or more.

What’s more, these results aren’t perfect, warns Michael Mina, a former assistant professor of epidemiology at the Harvard Harvard School of Public Health. T. H. Chana, and now Chief Scientist at eMed, a home testing and treatment company. PCR tests that look for genetic material from the coronavirus can also give a false negative result if you had a low viral load at the time. When in doubt, isolation for at least five days, as recommended by the US Centers for Disease Control and Prevention (CDC), can help prevent the spread of COVID-19. Mina even extended that five-day period to eight days, just to play it safe.

“There is a big difference in how quickly people get rid of the virus,” he says. “Some people clear it in four or five days; some people will need 15 days.”

Read more: New test could help determine if you’re immune to COVID-19

During the travelling

Now that mask mandates for air and other vehicles have been lifted, COVID-19 is potentially easier to transmit on planes, buses and trains than when we all covered our faces. Pre-travel testing is one way to keep an eye on others and make sure you’re not carrying the virus in an enclosed space.

This, of course, can help protect your fellow travelers, but this does not mean that everyone is so careful, and you may not be the one who spreads the disease, but the one who catches it. For this reason, del Rio takes tests with him when he is on the road. “When I travel, I check myself two to three days after arriving at my destination,” he says. “Then I’ll do the same when I get home.”

Mina, the father of a girl who is not currently vaccinated, checks the family carefully before traveling, especially if they are visiting other people in their homes. “If we’re going to be in someone’s home, we just don’t want to be the ones responsible for spreading COVID,” he says.

When communicating

Before the pandemic, no one gave much thought to the health implications of a dinner party, but now that has changed. Del Rio has made it a habit to check himself before gathering with a large group of people, especially if it’s indoors. There are more opportunities to be outdoors during the summer months, but SARS-CoV-2 can spread outdoors, although much less efficiently than indoors.

If you’re the host of a social event, things can get a little more delicate, which raises the question of whether it’s worth asking your guests to take a test as a kind of entry ticket to your home. Here del Rio does not take risks. “Suppose we have 10 or 12 people in our house: we will probably do some testing,” he says. “I would make the tests available right before they arrive.”

Mina agrees. “We let people come and don’t make it big,” he says. “We’re just saying, keep the mask on and check right before you go in, and then just leave it on for 10 minutes. We all feel much more comfortable knowing that everyone is negative.”

Visiting vulnerable populations — immunocompromised people, the elderly, or unvaccinated infants and young children — is another area where experts agree on testing protocols. Mina’s parents are in their 70s, and like del Rio and his 87-year-old mother, he checks on them before visiting them. Breeze conducts tests before visiting any person with a medical condition, even if he does not know if the person is immunocompromised. And while vaccines are currently available for children as young as 6 months of age, vaccine uptake is likely to be slow, and it is a careful precaution to test before visiting any child.

Read more: Research shows dogs can sniff out COVID-19 and signs of long-term COVID

During bursts

As the number of COVID-19 cases rises, special vigilance must be exercised. During the latest surge, Mina and his family were tested on average once a week, even if no one was symptomatic.

Breeze considers children to be a special area of ​​focus, as they spend their days at school in the company of so many other children and generally have extensive social contacts. Even if there are no known cases of COVID-19 in a child’s social circle, the risk of transmission exists, especially during a spike. “It might make sense to test children on a more regular basis,” he says.

However, if you have a limited number of tests, you may not need to test immediately after known or suspected contact with an infected person. “If you’ve just been exposed, don’t even bother testing for two days,” says Mina. “Wait at least as long, but often you have to wait three days, if not four, after exposure [to get an accurate result] because the virus must have a chance to become detectable.”

Ultimately, experts agree, testing is a personal decision and people should find their own level of comfort and risk tolerance. However, the goals should remain the same for everyone: to prevent infection and transmission of the virus to others. Masking and getting vaccinated on time are key elements in this COVID-19 toolbox. Express testing should also be one of them.

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Write Jeffrey Kluger at [email protected]

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