Why Rapid COVID-19 Test Results Are Getting More Confusing

After a recent COVID-19 exposure, Dr. Christina Astley tested positive on an at-home test—but just barely. The line signifying a positive result was so faint that Astley, an endocrinologist at Boston Children’s Hospital, took a picture and applied a camera filter to confirm it was there at all.

Further complicating matters, Astley later tested negative with a different manufacturer’s kit. Even for a physician who is “hyper-vigilant” about COVID-19, Astley says, the results were hard to interpret.

Experts say ambiguous results like these may be more common now—but not because rapid tests aren’t working. In fact, these confusing results could actually be a good thing, at least as far as your immune system goes.

When a rapid test is clearly positive, with a dark line showing up almost immediately, that means there’s been “a failure of the immune system,” says Dr. Michael Mina, chief science officer at home-testing company eMed and a former Harvard University epidemiologist. A blazing positive essentially means your body is “letting the virus get out of control,” he says.

At this point in the pandemic, when most people have been vaccinated multiple times and infected at least once, our immune systems are getting better at responding to the virus before it reaches that point, Mina says. That means exposures may result in shorter illnesses, if they lead to positive tests at all, he says.

“Your body has the head start instead of the virus,” says Shane Crotty, who studies infectious-disease immunity at California’s La Jolla Institute for Immunology. “It doesn’t take you seven to 10 days to catch up to the virus.”

As our immune systems get better at fending off SARS-CoV-2 through boosters and prior infections, Crotty says people should be ready for more faintly positive results, which could be shortly followed by negative tests as the body quickly clears the virus. People may also experience symptoms sooner after an exposure than they did earlier in the pandemic, Crotty says. Symptoms are a sign that your immune system is fighting back—so when the immune system knows what to do and responds right away, sickness often isn’t far behind.

It can take time for at-home tests to catch up to symptoms. That may be because the body is keeping the virus mostly in check, so there isn’t much of it in your nose when you take a swab, Crotty says—although that’s still an open question.

“A faint line of positivity can either occur if it’s too early in testing, before somebody has peaked with their infectious viral load, or it’s happening at the tail end of their infection,” says Dr. Paul Drain, an associate professor of global health, medicine, and epidemiology at the University of Washington who has studied rapid tests.

Moving forward, all of this means that “you might have to squint a little harder at your antigen tests,” Crotty says. “It’s not going to be this crazy bright band that shows up in 30 seconds, which is a sign that there’s massive amounts of virus in your body. And, big picture, that’s largely a good thing.”

But it can also be confusing. Does a faintly positive result, shortly followed by a negative, still mean you should isolate? What if you’re feeling sick but testing negative?

If you have COVID-like symptoms, the safest thing is to stay home and away from others, even if your tests are coming back negative. And if you tested positive, no matter how faint the line, you should act as though you have COVID-19, Drain says. “A positive is a positive,” he says.

Under U.S. Centers for Disease Control and Prevention (CDC) guidelines, that means isolating for five days. Then, for an additional five days, the agency recommends wearing a high-quality mask if you’ll be around others. The CDC says you can shed your mask early if you get a pair of negative antigen test results, 48 hours apart.

Mina agrees that you can feel confident you’re no longer contagious after a pair of negative at-home tests. Studies have shown that, at least with the variants we’ve seen so far, antigen test results correlate with your ability to transmit the virus. In other words, a positive result means you could be infectious, while a negative result means you probably aren’t.

As test results get more ambiguous, however, Astley worries that people won’t have the time and desire to take repeated tests and analyze whether there’s a faint line on the strip. Pale or short-lived positives may be signs that your immune system is getting better at handling the virus. But from a public-health standpoint, she says, they’re complicated.

“It is a double-edged sword,” Astley says, “because it means that what somebody was using to rely on for decision making” isn’t as clear-cut as it once was.