By now, with the many waves of Covid-19 infections we’ve endured, you may know someone who’s tested negative on an at-home lateral flow Covid test only to test positive in a doctor’s office.
This phenomenon – which isn’t uncommon, anecdotally speaking – has stumped experts. Could the tests be getting less precise as the coronavirus evolves? Or are there problems in the ways or frequency with which people test themselves?
Scientifically speaking, the tests generally are fine. Molecular biologists, the people who investigate diagnostic tests, don’t expect XBB — the latest variant creating a storm of infections across the US and beyond— to have much of an effect on the accuracy of our at-home rapid tests.
The subvariant XBB is just another version of Omicron and structurally looks pretty similar to all the Omicron subvariants that came and went in 2022, says Dr Wilbur Lam, a professor of paediatrics and biomedical engineering at Emory University and Georgia Tech and a physician at Children’s Healthcare of Atlanta.
To his team, this is good news when it comes to our rapid tests. “The way the tests have performed with previous Omicron subvariants, we would expect that they would perform very similarly with XBB,” Lam tells HuffPost.
Here’s why XBB probably isn’t going to impact the accuracy of at-home tests
There aren’t any studies or pre-print reports currently available on how effective rapid tests are with XBB, but scientists predict that at-home tests will continue to work well with the latest crop of variants.
The part of the coronavirus that at-home tests are designed to look for — the nucleoprotein, or the N-gene — hasn’t changed much, even as the virus has evolved and picked up a ton of mutations, according to Dr Nathaniel Hafer, an assistant professor of molecular medicine at UMass Chan Medical School.
Whenever we hear of new variants having concerning new mutations, those changes predominantly take place in the spike protein, which is the part of the virus that latches onto our cells, causing an infection, he says. But the N-gene has pretty much stayed the same with all the Omicron subvariants, including XBB.
While these mutations have made the coronavirus more transmissible, they haven’t really made it harder for diagnostic tests to detect the virus.
“If we look at the sequence of the different variants that are out there right now, I would predict that the at-home tests, or over-the-counter tests, should be continuing to perform at the high level that they have been in the past,” Hafer says.
Expect the rapid tests to continue to be at least 80% accurate, if not more. (They won’t catch 100% of infections – especially if tests are not conducted correctly – but they’ll come pretty close.)
Other factors could cause false negatives on at-home Covid tests
First, repeat testing is crucial, and taking a single test isn’t the most reliable way to determine if you’re infected. If you have symptoms, you should test yourself at least twice 48 hours apart; if you don’t have symptoms, do three tests 48 hours apart.
“As the infection progresses from day to day, then you start increasing the amount of virus in the nose and that’s ultimately what determines whether a test is able to catch a positive or negative,” Lam says.
There is such a thing – especially with these newer strains of the virus – as testing too early. There has to be enough virus in your body for at-home tests to work. And with the Omicron subvariants, there doesn’t seem to be as much virus in the nose at the beginning of the infection, Lam says.
Keep in mind that evidence suggests at-home tests aren’t super useful for asymptomatic cases: A report published in November found that rapid tests only pick up about 27% of asymptomatic infections.
Researchers suspect that is because asymptomatic people may not have enough virus for the test to catch the infection – which brings us to the next theory.
People who test negative on an at-home test but positive at the doctor’s office may just have a low viral load. PCR tests or more sensitive lab tests conducted at a doctor’s office can detect lower levels of virus compared to the at-home tests. This possibility is looking more and more likely, especially as we enter the era in which the vast majority of people have either been vaccinated multiple times or infected, also potentially multiple times.
“There might actually be less viral stuff in the nose because the immune system is doing its thing,” Lam says.
Another lesser-known theory: Omicron, including XBB, may interact with our cells differently and shed less viral protein in the nose compared to previous variants, according to Lam. This isn’t well understood — but, at the end of the day, how much viral protein is in the nose is what causes a rapid test to produce a positive test result and there may be something inherently different about the Omicron subvariants, Lam says.
When should you go to get tested if you keep testing negative at home?
If you have symptoms or were exposed to someone with Covid but continue to test negative – even if you’re repeat-testing and swear you’re getting a good swab – you may wonder when it’s time to get a PCR test. The short answer: it really depends on your situation.
When Lam gives advice to his family and friends, he asks, “Why do you want to know?” If you are at risk or if you live with or are in close contact with someone at risk – such as someone who is elderly or has a chronic illness — it’s probably worth getting a PCR test sooner rather than later so you can start treatment, isolate yourself and let others know they might want to test.
If you’re not at risk and are doing fine with mild symptoms at home, Lam recommends waiting a couple of days, then checking in with your doctor if you still feel crummy. No matter what – even if your symptoms are mild and you feel like you’re recovering – getting tested at a clinic or pharmacy can’t hurt, especially before being around other people.
And if you’re getting worse, it’s definitely worth setting up an appointment with a health care provider, Hafer says. Though you may find out you do, in fact, have Covid, there are many other viruses going around right now — including the flu, RSV and the common cold – that are making people sick. A health care provider can run a respiratory panel to determine what’s causing your symptoms, and depending on what you have, prescribe treatments and get you on the path to recovery.