Rapid Tests Don’t Always Spot Omicron. How to Know You Can Trust Result
- Early indications suggest rapid tests work on Omicron, but only if you take them when your viral load is high.
- You need to reach a certain threshold before any test — PCR or rapid — can detect your infection.
- Experts recommend isolating first, then taking at least two rapid tests, spaced a day or two apart.
Rapid tests for COVID-19 are simple to administer and ready within minutes, involving little more than a quick swab of your nostrils plunked into a solution of virus-hunting chemicals.
But if you don’t perform rapid tests in the right way at the right time, they won’t provide accurate results.
With the federal government promising to distribute half a billion free COVID-19 tests across the country in 2022, and set to reimburse over the counter pharmacy kits starting in January too, many people will use free rapids next year.
So if your resolution is to test cheaper and smarter in the new year, here’s a primer on the best ways to use rapid tests to yield the most useful results.
You can’t always trust a result within the first 2 days of exposure
If you test yourself too early during the course of an infection, your test will turn up negative, regardless of whether you’ve actually got COVID-19 or not.
“There’s a misconception if you’ve been exposed, you should get tested right away,” Susan Butler-Wu, who directs clinical testing for infectious diseases at the University of Southern California, told Insider. “There’s absolutely no utility to doing that if you just got exposed, because the virus needs time to replicate.”
Rapid tests are designed to yield positive results when your viral load is highest, and that takes at least a few days. During that time, you may or may not develop symptoms of Omicron, which can include cough, fatigue, and a runny nose.
“If it were me and I were symptomatic starting today, I’d probably quarantine myself,” epidemiologist Michael Mina, chief science officer at eMed, said during a call with journalists on Tuesday. “I would take the test tomorrow, and I would take the [second] test in two or three days after that.”
Even if your first test is negative, it’s possible you could already be spreading the Omicron variant around.
“Especially right now with such high incidence across the country, if you feel any symptoms, assume you have Omicron,” Mina said, adding he’s already canceled his holiday travel plans. “Don’t be scared, you don’t have to be extremely worried about it, but just assume it’s Omicron and quarantine. And at that point, you can then take another test.”
Keep in mind: A low viral load (and thus, a negative rapid test) do not necessarily mean you’re safe to mingle mask-free.
“If I have less virus, but now I’m sitting in a room with no windows open, in close proximity to somebody who’s unvaccinated, I may still be infectious,” Butler-Wu said. “I might be less infectious, but I can still transmit.”
If you’ve been exposed, isolate for 3-5 days — then test
Experts say the best time to administer a rapid test is generally somewhere between 3 to 5 days after an exposure to COVID-19. If you did get sick, that waiting period will give the virus enough time to start replicating inside your body and reproducing itself, making it more detectable on the test.
Most rapid tests come in a two-pack. Take one shortly after your symptoms start, and save the other for 24-48 hours after the first, Butler-Wu says, following the package instructions carefully.
And, she says, don’t “randomly start swabbing various things” like your throat unless that’s what the test actually calls for (in theory, it’s possible that drinking acidic substances like coffee or soda could lead to false positive throat swabs.)
PCR tests can detect the virus earlier, but experts still recommend waiting a few days
In general, higher-quality PCR lab tests for COVID-19 (also known as molecular tests) pick up lower levels of the virus more readily than rapids do.
That means they can detect the virus earlier in the course of your infection — generally, they can turn positive about a day before a rapid test.
Still, it’s best to wait several days after an exposure for your PCR, too, to really ensure an accurate result.
“If you’re doing a PCR and you do it at day five, that’s pretty good,” Butler-Wu said.
Because of their enhanced viral sensitivity to the virus’s genetic material, PCR tests will remain positive far longer than rapids do during the course of an infection, and they may continue to detect the virus for several weeks after a person feels better.
“People like to say it’s a ‘false positive,’ but it’s not a false positive,” Butler-Wu said of the long tail on PCR test positivity. “You had the virus at some point — we’re still detecting remnants of it in a way that we don’t detect with the [rapid] antigen test.”
If you’re planning to mingle, rapid test just as you arrive, not hours before
Scientists are still zeroing in on when people are most likely to be infectious during the course of their Omicron illnesses. In the meantime, experts stress an old-fashioned, common sense approach to disease prevention is best, with this highly transmissible variant on the loose.
“For the holidays, if you’re symptomatic, just don’t go,” Mina said. “Always assume a symptomatic person’s positive.”
Remember, getting a negative test right before an event can’t protect you from the virus while you’re there.
“It only takes one person to enter into a party and infect a lot of people,” Mina said, recommending cutting down on holiday travel this year if you can (he’s not flying), and keeping any gatherings you do have small. “Me testing negative before I walk into a party is not going to help me not become positive if somebody’s breathing on me in that party [who has COVID-19].”
Butler-Wu agrees that getting a negative test doesn’t ever mean you’re 100% safe.
“It’s not an infectiousness test,” she said. A rapid test shows you “what’s going on in your nose at that moment,” but many other factors, including the immune status of both the infected and infecting person, ventilation, vaccination, and duration of exposure, can all play a role in how well the disease spreads.
‘One layer of reducing risk’
The good news is that so far, rapid antigen tests seem to identify the new variant well, when timed right. Rapid tests home in on the coronavirus’s neucleocapsid protein, which has just four mutations with Omicron. Early indications from the Food and Drug Administration indicate rapid antigen tests are decent at detecting all of the currently circulating coronavirus variants, but rapids aren’t perfect.
“It needs to be seen as one layer of reducing risk, rather than it being the layer that eliminates risk, which is sometimes, unfortunately, how I think it’s portrayed,” Butler-Wu said.
Early studies from the UK back up what the FDA has found so far, suggesting rapid tests do what they were designed to for Omicron. Still, they have to be taken at the right time, and ideally, performed multiple times over the course of a few days (right when a person’s viral load is highest) for the very best results.
“What we’re seeing is on a per viral particle basis, the rapid tests are working as well today as they did with Delta,” Mina said.