This Is What You Should Know Before Taking A Rapid COVID Test

Every day, decisions are being made based on rapid COVID tests. How common are false negatives, and can you trust the results?

A rapid antigen test
Joseph Prezioso / AFP via Getty Images

If you’ve tried to buy or get a COVID test lately, you may have found long lines, empty shelves, and a frustrating lack of availability in general.

This is happening as Omicron is making up the majority of new cases in the country, now about 95% of cases in the US.

President Joe Biden recently announced that the US government is buying 500 million at-home COVID tests for the nation.

While increased test availability is a good thing, those government-supplied, at-home COVID tests may not be available to request until January and could take months to be distributed. And if you are able to get your hands on rapid COVID tests, a lot of questions remain. How reliable are they? When should you use them? Do they perform differently with Omicron?

Here are some important things to know about at-home COVID tests.

When should you use rapid COVID tests?

Rapid COVID tests are, well, rapidly becoming a way of life in the US. After being absent in the US for much of the pandemic, they are more widely available — and often sold out — in pharmacies and grocery stores as well as online.

The pro is that they give fast results — in about 15 minutes, allowing you to make quick changes in behavior if you get a positive result.

The con is that they may not be as accurate very early in an infection compared with other tests, like a PCR, that can take a day or more to get results.

“Rapid tests are one of the most powerful tools that have not really been utilized in a powerful way in this pandemic,” said longtime testing advocate Dr. Michael Mina, a former Harvard epidemiologist who is now the chief science officer of eMed, a digital platform that facilitates at-home testing. “The government is in a position now where it’s bringing these tests forward, and now is the time to start creating a strategy around how will they be most effectively used,” he said Tuesday at a press briefing.

You can take a COVID test for any reason at all, but here are the times the FDA recommends you get tested, even if you have been vaccinated.

  • If you have COVID symptoms, including cough, shortness of breath, fever, and other respiratory virus symptoms
  • Have been within 6 feet of someone with a confirmed case of COVID for a total of at least 15 minutes. (If you are unvaccinated, get tested right away; if you are fully vaccinated, get tested within five to seven days after exposure, according to the CDC.)
  • If you took part in high-risk activities, including any time you couldn’t socially distance as recommended, such as when traveling, in crowded indoor places, or attending large gatherings or mass events

Many people who are asymptomatic are now getting screening tests for COVID at work, school, or before or after travel or events.

Although vaccines, especially when combined with a booster, are likely to protect against hospitalizations and death due to the Omicron variant, “we know we will continue to hear more about people who get infected who were vaccinated,” CDC chief Rochelle Walensky said at a press briefing on Wednesday. “These people may get mild or asymptomatic infections and could unknowingly spread those infections to others.”

Regardless of vaccination status, you should wear a mask in public indoor settings. Walensky encouraged people to take at-home COVID-19 tests before gathering indoors “to help protect you and your family and friends who may be at greater risk of COVID-19 or severe outcomes.”

How many different at-home COVID tests are there?

There are now (at least) 11 over-the-counter antigen tests, which are the rapid tests that you can buy, take yourself, and generally cost about $20 to $35 for two — if you can find them.

An antigen test detects proteins produced by the SARS-CoV-2 virus, which can be picked up with a nasal swab.

Keep in mind that no test is perfect. Any given medical test is measured by its sensitivity, or how good it is at picking up actual cases and not generating false negatives, and specificity, how good it is at ruling out people who aren’t sick and not generating false positives.

Here are some of the at-home antigen tests that have been approved via the FDA’s emergency-use authorization:

  • BinaxNOW COVID-19 Antigen Self Test
  • iHealth COVID-19 Antigen Rapid Test
  • Flowflex COVID-19 Antigen Home Test
  • BD Veritor At-Home COVID-19 Test
  • CareStart COVID-19 Antigen Rapid Test/On/Go COVID-19 Antigen SelfTest
  • SCoV-2 Ag Detect Rapid Self-Test
  • InteliSwab COVID-19 Rapid Test
  • Celltrion DiaTrust COVID-19 Ag Home Test
  • Quidel QuickVue At-Home OTC COVID-19 Test
  • Ellume COVID-19 Home Test

There are also over-the-counter molecular tests that are similar to PCR and can detect the virus earlier than an antigen test, but they cost more, Mina said. They include Detect, Cue Health, and Lucira, which can be done at home, as well as other molecular tests that are done in doctor’s offices.

“Rapid molecular tests are slightly more sensitive but they are a lot more expensive,” Mina said. There are also fewer of them available. “It’s a much more difficult technology to build than a rapid antigen test,” he said.

Are rapid COVID tests accurate and reliable?

In general, yes. But you should know that false negatives are more likely to occur early on with rapid tests than with laboratory tests, like PCR.

And it’s important to note, a small new study suggests that you can spread the virus in those early days if you are getting a false negative on a rapid antigen test.

The study, which is not yet peer reviewed, included test results from 30 people out of hundreds involved in workplace outbreaks in New York, Los Angeles, and San Francisco in December 2021. The people were fully vaccinated, many with boosters.

The research team, led by epidemiologist Blythe Adamson, compared nasal swab rapid antigen tests to saliva-based and nasal-swab-based PCR test results. They found that at least four people had transmitted the virus during the time they had a false negative on a rapid test.

While it’s “alarming” that the rapid tests didn’t always detect a high viral load, the study is getting a lot of attention and manufacturers will be working to update their tests, Adamson told BuzzFeed News.

“FDA is paying close attention and I think manufacturers are ready to try to adapt and be agile,” she said. “So I think it will move quickly.”

False positives are much less common. However, the chances of a false positive can vary by brand, ranging from one in 150 tests to one in 5,000, said Mina.

That said, if your rapid test gives a positive result, you should assume you probably have COVID and isolate until you follow up with another type of test, like PCR.

Rapid tests are about 30% to 40% less sensitive than PCR tests, and they are more accurate in people with symptoms than without symptoms.

In one study, a popular at-home rapid test had a sensitivity of about 35% in asymptomatic people and 64% in people with symptoms when compared with PCR, but was around 100% accurate when it came to people who tested positive for COVID, whether or not they had symptoms. In the study, there were only a handful of false positives, all in asymptomatic people — but 47% were false negatives compared with PCR.

These are still good tests! It’s not fair to compare rapid tests you can do at home in 15 minutes to a lab-based result that might be more accurate but takes a day or more (often many more) to get a result, Mina said. You may just need to keep all that in mind when taking a rapid test and know that taking multiple rapid tests is part of the strategy.

“The test is very good when it’s positive, but when negative may frequently give you a false negative result,” said Dr. Stanley Weiss, a professor of biostatistics and epidemiology at the Rutgers School of Public Health in New Jersey.

If you get a negative result, follow up with additional rapid tests in a few days to see if it becomes positive, especially if you have reason to believe that you were exposed or infected. Wait at least 24 hours for your next test, according to the CDC, but retesting a few days later can help you feel comfortable that a negative result is a true negative.

Mina recommends that if you start having symptoms, assume you have Omicron and self-isolate. Take your first rapid test 24 hours after symptoms start and test again two to three days later.

When interpreting and making decisions based on a test result, it’s a good idea to take into account a person’s history of vaccination, mask use, exposures to others, and their general likelihood of actually being COVID positive, Weiss said.

Can rapid tests detect the Omicron variant?

Yes, that seems to be the case, although the FDA recently announced that rapid antigen tests are less sensitive to Omicron than the Delta variant.

It makes sense that a variant might change the reliability of the tests used to detect the virus and the FDA is asking manufacturers to test their products to make sure they work against Omicron and future variants.

The FDA has identified a few PCR-based tests that don’t and have recommended that labs don’t use them.

If you do have COVID symptoms, can it now take longer for tests to show a positive result than at the beginning of the pandemic?

Yes, according to Mina. That’s because early in the pandemic, humans had not been exposed to the virus previously and symptoms — which are a sign the immune system recognizes and is fighting the virus — started later in the timeline of infection.

Now that millions of people have either had COVID, been vaccinated, or in some cases both, symptoms generally start sooner. (And it’s clear that you can get COVID two or more times.)

“You're going to become symptomatic potentially within a day after exposure because your immune system kicks in so early,” Mina said. He also noted that’s why COVID symptoms have changed and now include congestion and runny nose. “That’s your immunity working,” he said.

Is a lateral flow test the same thing as an antigen test?

Not exactly. A lateral flow test is a specific type where you dip a piece of paper in a liquid and after a few minutes one line appears — a positive control to show the test is working — and another line appears if the test is positive.

While all rapid antigen tests currently on the market for COVID are lateral flow tests, not all lateral flow tests are COVID tests. For example, pregnancy tests also work in this way but use urine rather than a nasal swab for testing.

Can a rapid test protect you from getting sick?

Not really. Rapid tests are more about protecting other people, not making sure you don’t get the virus. (That’s what vaccines, masks, and social distancing are all about.)

However, when people get tested before going to work, a party, school, or traveling, it helps protect everyone. (Although it also helps you get treatment sooner, which is important because almost all effective treatments for COVID are more likely to work the sooner they are started.)

Rapid testing is particularly good to help stop superspreaders of the virus, Mina said.

“For somebody who’s really spewing out tons of virus and is very likely to infect a lot of people, then the tests are more likely 95% or 98% sensitive for those individuals,” he said.

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