Covid testing: What to know about accuracy for at-home, rapid and PCRs for the Omicron variant

December 23, 2021, 6:27 PM UTC

There’s a hot holiday gift this year—but it’s not a toy or an X box. What your nearest and dearest want most this year is likely a stash of at-home COVID tests. Though they were relatively easy to find just a few short months ago, the sudden emergence of Omicron at the same time millions of people were gearing up to travel for the holidays has created a perfect storm for anyone hoping to score a test. That’s meant strict limits on how many you can buy at a time, sold out shelves across the country, and more questions than answers for would-be travelers wondering if it’s safe to do so. Here’s what we know about the state of testing right now.

Where can you buy a test?

In areas currently experiencing a surge, like the New York metro area and the Midwest, at-home testing kits are few and far between. Some retailers, in an effort to maintain inventory, have instated a per-customer limit. This week, Walgreens announced it would be limiting customers to four test boxes each, while CVS limited to six, and Amazon, where tests are in and out of stock, has a limit of ten.

Stores including Walgreens, CVS, Rite Aid and Walmart all offer up-to-date inventory information on their respective websites, where customers can find the closest chain location with rapid tests in-stock, or potentially order boxes online. Other cities, like Washington, D.C., are offering free testing kits at local library branches, though pre-holiday lines have stretched into the hundreds.

Ideally, getting a rapid test when you need one will soon be much easier. President Biden said this week the government would buy 500 million rapid tests and distribute them, free of charge, by early next year.

Is a rapid test as good as a PCR?

When used correctly, almost.

Rapid test results come in minutes, while PCR tests can take days, and require lining up at testing site or healthcare facility with other potentially ill people. PCR tests make many copies of the virus’s genetic material, allowing it to detect even the smallest traces of the virus. Rapid tests, on the other hand, don’t amplify the virus, making them less sensitive. 

If a rapid test is taken too soon after infection, the virus may not yet be widely replicated, meaning the test won’t pick it up and will show a false negative. This means rapid tests are best used for those with symptoms; their real-world performance is “generally lower” than PCRs, according to the CDC. Some recent research indicates that Omicron’s incubation time is shorter than earlier variants, and symptoms can show up three days after exposure, versus closer to five for previous strains.

Your best bet at getting a PCR-level accurate result with a rapid test is to take a rapid every three days, which will ensure you catch different viral load levels. Using rapid tests this way will catch 98% of infections, according to the NIH, putting them on par with what they call “the gold standard”: PCRs. 

Can at-home tests detect Omicron?

Yes, mostly. On Wednesday, the FDA confirmed BinaxNOW and QuickVue, two of the antigen tests most commonly sold in stores, are able to detect the omicron variant with similar performance as with other variants. The same goes for most PCR tests, which the agency said should not be significantly impacted. However, there are no authorized COVID-19 antigen or molecular tests that specifically report the presence of a given variant; they’re only designed to check broadly for the SARS-CoV-2 virus, which includes Delta and Omicron.

Why is there a shortage of rapid tests?

A few factors are to blame. The holiday travel season coincided with the highly-transmissible Omicron variant, leading more people to scramble for tests.

But as Fortune wrote earlier in December, “On the supply side of the equation, the federal government and test manufacturers have been working together to scale up inventory over the past two months. Much of the blame for low test supply has been placed on the FDA, which has authorized just 14 rapid antigen tests for COVID-19. Compare that to the 46 available in Europe. According to reporting by ProPublica, the agency has moved painstakingly slowly in evaluating EUA applications for rapid tests, many of which have already been approved in other countries. One FDA scientist even quit in protest over the delays.”

Additionally as Fortune reported, many of the at-home tests are never making their way to consumers at all, but are instead going straight to businesses and other organizations that have placed large orders to test their own employees. As we wrote, “For example, Intrivo, which partnered with Access Bio to produce the rapid test On/Go, recently announced deals with Publix, Norwegian Cruise Line, and other Fortune 500 companies. The same tests are also available direct to consumers through Walmart and Amazon. Intrivo co-CEO Ron Gutman says that right now, a higher percentage of the tests they produce are going to business clients rather than individual customers, but he emphasized that the company is working hard to meet both sides of the demand.”

Why do at-home COVID tests sometimes show false negatives?

Rapid tests work best when you’re actively experiencing symptoms; meaning when you are shedding the most virus particles, and the tests are most likely to pick them up. These symptoms, particularly with the latest Omicron variant, often take several days to kick in. This is why a recently exposed person’s rapid test may show a false negative, only for a PCR or additional rapid taken a few days later to show positive.

This is why the NIH recommends taking rapid tests every three days after coming in contact with a sick person; it ensures you’ll catch different viral load levels. Using rapid tests this way will catch 98% of infections, putting them on par with what they call “the gold standard”: PCRs. 

When should you test after potential exposure?

Fully vaccinated people should get tested 5 to 7 days after their exposure, the CDC says, even if they don’t have symptoms. It can take several days for enough virus to build up in your system to show up on a PCR test, meaning taking a test too soon could result in a false negative.

What should you do while you’re waiting for PCR results?

If you’ve been exposed or are feeling sick, isolate. You may pass the virus onto others before the classic COVID-19 symptoms develop, and staying home is key to slowing community spread.

The CDC outlines a three-step approach to this critical period. The first is to stay home and monitor your health. The second is to consider anyone you’ve been in contact with, in case you need to let them know they’ve been exposed. The third is to answer calls from your local health department, who may bring news that you yourself have been exposed.

Are more at-home test coming?

According to President Biden, yes. He is prepared to supply 500 million at-home tests for free to Americans that request them beginning in January. They’ll be available through a website, but details have not been released yet.

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