Abbott Laboratories BinaxNOW, an at-home-rapid-COVID test, two-pack cost $23.99. (Hannah Norman/Kaiser Health News/TNS)

(TNS) – With COVID-19 cases surging across the San Francisco Bay Area, many people are using at-home antigen tests, especially as they become more widely available from employers, schools and more. Antigen tests aren’t as reliable as PCR tests, but they’re likely to catch anyone who’s at their most contagious.

One thing many people using at-home COVID-19 antigen tests are getting wrong is the timing, said Dr. William Schaffner, a professor in the division of infectious diseases at Vanderbilt University. Schaffner explained that he has noticed people are often testing too early after exposure and without any symptoms, and this can lead to false negatives.

“I think there are many people who are using the at-home test within 48 hours of being exposed to someone,” he said. “They’re thinking, ‘I’m exposed. It’s now two days later and I want to test.’ That’s too early. There’s not enough virus there yet. You have to let enough time go by.”

How to determine when to test for COVID after exposure

If you feel sick and have symptoms, the Centers for Disease Control and Prevention recommends that you test immediately, even if you’re vaccinated. You’re also advised to test if you had close contact with someone who tested positive for COVID, even if you don’t have symptoms. If you’re fully vaccinated, general health guidance recommends testing on day 5 after you were exposed.

Schaffner said you can use an at-home test as early as day 3. This is because the incubation period for omicron, which is the dominant variant in the U.S. and California, appears to be shorter than past variants. Symptoms can start showing up as early as three days after exposure, the Centers for Disease Control and Prevention said.

But Schaffner emphasized if you’re going to test on day 3, you have to count correctly.

“You don’t count the day of exposure,” he explained. “If you’re exposed on Monday, Tuesday is day 1, Wednesday is day 2, Thursday is day 3.”

He added, “I think miscounting is often a mistake. It’s not people trying to cheat the system. It’s just people who don’t know how to count.”

When people count the day of exposure as day 1, they are likely to test too early when there may not be enough virus present. The at-home test, which is less sensitive than a PCR, may not pick it up. If you test negative on day 3, you can test again on day 4 or 5, depending on your situation and access to test kits. If you have only one test, you may want to wait to test on day 5, as Schaffner said.

“There’s a fair amount of biological variation,” he said. “Some people will turn the test earlier. Some will take longer.”

Swab the interior of your nose vigorously for an accurate at-home COVID test

Schaffner said another mistake people are making is not being aggressive enough with the nose swab.

“You do have to swab the interior of the nose vigorously,” he said. “There are some people who don’t like to fool around with their nose. If you want an accurate result, you really have to fool around in there. Don’t just touch it gently. You’re trying to rub off the mucous membrane.”

Some small studies have suggested that taking a throat swab may increase the chances of picking up the virus, especially earlier in the infection. Schaffner noted that while this could be true, it’s difficult for people to do their own throat swabs.

“I would think if a throat swab is going to be used, a lot of people would need somebody to get it for them” he said. “You need a doctor or a nurse or somebody who knows what they’re doing. If you’ve ever had a throat swab done correctly, you’ll gag.”

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