Rapid tests do not always detect Omicron — here's how to know for sure if you've got COVID-19 or not

• Rapid tests work well on Omicron variants, but only if your viral load is high at the time.
• 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 dunked into a solution of virus-hunting chemicals.

Last year, the federal government promised to distribute half a billion free COVID-19 tests across the country in 2022 and to reimburse over-the-counter pharmacy kits starting in January.

The program has now been brought back to life after being paused since August, meaning US citizens can request four at-home tests per family. Americans can also still get reimbursement from the government for 8 at-home tests a month. 

But if you don't use rapid tests in the right way at the right time, they won't provide accurate results. And some experts have expressed surprise at how infectious Omicron has been over the last year, with symptomless, vaccinated people shedding the virus and infecting others days before a rapid test detects their infection.

According to early studies from the UK and the US Food and Drug Administration, rapid tests still work. But 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 best results.

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 get 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 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."

In fact, the CDC currently only recommends testing immediately if you have symptoms. If you're exposed, the CDC says to wait at least five days. If you get a negative result, test again in one-to-two days. 

Rapid tests are designed to yield positive results when your viral load is highest, and it takes at least a few days for it to get to that level. 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," Michael Mina, an epidemiologist and the chief science officer at eMed, said during a call with journalists last year. "I would take the test tomorrow, and I would take the 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.

Tim Spector, the epidemiologist who runs the highly regarded Zoe COVID Study in the UK, told Insider he was surprised to find Omicron and newer variants seemed to move "much faster" than Delta and other older strains of COVID-19.

Early data he collected on a birthday party of fully vaccinated adults — who all took rapid tests before they arrived at the event — suggests someone can be infected with Omicron, not know it, and spread that infection to people who then go on to develop symptoms in just 48 hours.

"By the time people know they've definitely got it, they may have spread it," Spector said, adding that "the timing has to be just right" for tests.

It "makes track and trace pretty impossible," he said, and that's a good argument for keeping holiday parties very small (he canceled his).

Keep in mind: A low viral load (and thus, a negative rapid test) does 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 to 5 days — then test

Experts say the best time to administer a rapid test is generally somewhere between 3 to 5 days after 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, which makes 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 to 48 hours after the first, Butler-Wu said, and follow the package instructions carefully.

And, she said, don't "randomly start swabbing various things" like your throat, unless that's what the test calls for (in theory, it's possible that drinking acidic substances like coffee or soda could lead to false positive throat swabs).

With Omicron and its various offshoots being so transmissible, "if you feel any symptoms, assume you have Omicron," Mina said, adding that he canceled his 2021 holiday plans to stay safe from the virus. 

"Don't be scared," he said. "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."

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 exposure for your PCR test, 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' genetic material, PCR tests will remain positive far longer than rapid tests 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 antigen test," she added, referring to a rapid test.

If you're planning to mingle, take a 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 say 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. "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."

To protect yourself from falling victim to an Omicron exposure this holiday season, Mina recommends avoiding flights and large gatherings when necessary. But COVID-19 cases are largely down compared with this time last year, according to the CDC's case tracker. 

Butler-Wu agreed that a negative test didn't guarantee your safety. 

"It's not an infectiousness test," she said. A rapid test shows you "what's going on in your nose at that moment," she added, but many other factors, including the immune status of both the infected and the infecting person, ventilation, vaccination, and duration of exposure, can play a role in how well the disease spreads. 

'One layer of reducing risk'

The good news is that so far, rapid antigen tests have been identifying the new variant well when timed right. Rapid tests home in on the coronavirus' nucleocapsid protein, which has just four mutations with Omicron.

Early indications from the FDA indicated rapid antigen tests are decent at detecting all the circulating coronavirus variants, but they 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.