4 reasons your rapid COVID-19 test might 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.
Rapid testing has become a popular tool in the
In hot spots like New York City, where infections surged, many pharmacies reported selling out of take-home kits containing rapid antigen tests when Omicron was peaking. Lines were long at testing hubs and sidewalk tents alike.
Now, rapid tests are widely available. But the results you get from these tests aren't always perfect barometers of COVID-19 infections.
Rapid tests have always been second to polymerase chain reaction, or 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 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 often going for roughly $20 per two-test package.
Although it might be tempting to stock up on home COVID-19 tests, 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 an 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 2021 expired 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 of last year, after a review from the Food and Drug Administration. Other companies have issued similar announcements, and it's possible we'll see the expiration 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
Recommended temperatures for storage and preparation can vary among tests, so make sure to check the packaging first.
The BinaxNow kit, for instance, can be stored from 35.6 to 86 degrees Fahrenheit, 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 could cause false positives if you're using an oral swab.
"You're going to 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 — could "mess with the chemistry" of a rapid or lateral flow test. Butler-Wu clarified that this risk is pretty theoretical. One study found that false positives showed up when unexpected substances were directly applied to test kits.
But still, 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.
At-home tests detect the Omicron variant well
It was first believed that rapid tests might not be very good at detecting Omicron, a coronavirus variant that's — along with its many offshoots — demonstrated an ability to evade some of our immune defenses.
But, that's no longer the case.
According to a study from March, at-home rapid tests are "not inferior among individuals infected with the SARS-CoV-2 Omicron variant as compared to the Delta variant."
That is if you use the
The same review found that diagnostic tests from Applied DNA Sciences, Meridian Bioscience, and Tide Laboratories were more likely to return false negatives due to ineffective detection of the Omicron variant.
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