4 reasons your rapid COVID test may show a false result

• Rapid tests are a quick and convenient way to learn about your COVID-19 status.
• The timing, temperature, and even what you eat before getting tested could affect your results.
• Some rapid tests may fail to detect Omicron, but at least two existing test kits still work.

Rapid testing has become a popular tool in the coronavirus pandemic, especially during the recent Omicron wave.

In hotspots like New York City, where infections are surging, many pharmacies reported selling out of take-home kits containing rapid antigen tests. Lines are long at testing hubs and sidewalk tents alike.

However, the results you get from the tests that are currently available aren’t always perfect barometers of COVID-19 infections.

Rapid tests have always been second to PCR assays when it comes to accuracy. While antigen tests deliver results in about 15 minutes, before the Omicron variant emerged they were only 58% accurate for people who didn’t have symptoms, or 72% accurate for those who did, according to a Cochrane review of more than 24,000 test samples. 

In comparison, a correctly done PCR test can return lab-approved results with 98% accuracy, according to the same study based on pre-Omicron data. It just takes longer to hear back — and you’re supposed to self-isolate while you wait. 

If you opt for rapid results over the waiting game, here are a few things to keep in mind that could affect your test.

Home test kits have expiration dates

At-home COVID-19 test kits are one of the more convenient inventions to come out of the pandemic. Multiple companies offer rapid antigen tests over-the-counter, with take-home kits going for less than $30 a two-test package.

Although it may be tempting to stock up on home COVID-19 tests if you can find them, keep in mind that the kits expire within several months to a year.

Over-the-counter test kits typically have a sticker on the box indicating the expiration date and manufacturing date. The later date indicates the end of the test’s shelf life, which might be sooner than you’d expect — a BinaxNow test kit I bought at Walgreens in September is due to expire before the end of January 2022.

Abbott, the manufacturer of the BinaxNow kits, extended their shelf lives from six months to a year in May, following review from the Food and Drug Administration. Other companies have issued similar announcements, and it’s possible we’ll see the expiry dates pushed out further as the FDA continues to review stability studies.

In the meantime, users of home test kits should always assume a result from an expired test is wrong. 

Hot or cold temperatures can mess with test components

The temperature at which you store your home test kit matters. 

The little vials of liquid meant to be mixed with your sample are not designed to withstand extreme temperatures or humidity. 

“Don’t use rapid tests when it’s really cold, that can diminish their benefit or their efficacy,” Michael Mina, chief science officer of eMed, said on a call with journalists Tuesday afternoon. “You want to do it at room temperature for the most part.”

Recommended temperatures for storage and preparation may vary among tests, so make sure to check the packaging first.

The BinaxNow kit, for instance, can be stored at 35.6°F to 86°F, but a fine-print warning says to make sure all test components are at room temperature before use.

What you ate for lunch could affect a throat swab

If your COVID-19 test requires a throat swab instead of a nasal swab, what you eat before getting tested could also influence rapid results.

Susan Butler-Wu, who directs clinical testing for infectious diseases at the University of Southern California, told Insider that coffee and Coca-Cola can cause false positives if you’re using an oral swab.

“You’re gonna swab your mouth after you just drank your coffee, you’re gonna get a positive, potentially,” she said. 

Anything acidic — like coffee, soda, or fruit juices — can compromise the integrity of a rapid or lateral flow test. While the testing solution should neutralize any acids if mixed properly, the materials inside the test strip are too delicate to chance it.

Test sites and kits that involve oral swabs will warn you to avoid eating, drinking, and smoking for at least 30 minutes before the test. This advice might be familiar to folks in the UK, where rapid tests commonly require a throat swab in addition to a nasal swab.

You should also avoid brushing your teeth or using mouthwash before a throat swab, since good oral hygiene could temporarily wipe the virus out of your mouth and lead to a false negative.

We still don’t know how well the tests detect Omicron

Some rapid tests may not be very good at detecting Omicron, a coronavirus variant that’s already demonstrated an ability to evade some of our immune defenses.

Experts are in the process of checking whether existing rapid antigen tests maintain their accuracy of diagnosis for Omicron, Dr. Anthony Fauci said in a virtual briefing Thursday. Preliminary information suggests that not all of them will work for the new variant.

“We’re trying to find out those that don’t reflect an accurate result, and if we do, make sure that those tests are not used to diagnose Omicron,” the nation’s infectious disease doctor said in a virtual conversation with the Chamber of Commerce Foundation.

So far, the Abbott BinaxNOW and Quidel QuickVue antigen tests seem to detect Omicron as accurately as other variants, according to laboratory studies and FDA review. 

The same review found that diagnostic tests from Applied DNA Sciences, Meridian Bioscience, and Tide Laboratories were likely to return false negatives due to ineffective detection of the Omicron variant.

In time, scientists will have more data from the real world to help them understand how Omicron impacts test performance.