Why Even a Faint Line on Your Rapid Test Still Means You're COVID-Positive

Here's what the intensity of the line on your at-home COVID test can tell you about your illness.

Woman Holding a Covid-19 Rapid Test At Home
Photo: Getty Images

At-home COVID-19 tests provide a relatively simple and accessible way to see if you've become sick with the virus. But going through the testing and results on your own can be a bit confusing, especially with so many factors at play.

For those who do test positive, the time it takes for the line to appear on the COVID test and what that line might look like—from barely visible to bright and opaque—can vary. And though there isn't a ton of data yet available on the subject, experts say that, at least in this case, a darker or more faint line probably means exactly what you think it does.

"The heavier the line, the more virus there is. The fainter the line, the less virus there is," Peter Chin-Hong, MD, a professor of medicine and infectious disease specialist at the University of California, San Francisco told Health. "I know people have seen that line get fainter and fainter over time, and that's really what it is."

But even though the darkness of a positive line on a COVID test is a correlation of how much virus is in the body, it still isn't a perfect measure to tell a person how infectious or sick they might be. Essentially, a positive result is still a positive result, no matter how it looks—and you should follow the same COVID safety precautions, regardless.

Here's what we know about the factors that can affect whether you'll get a thick or faint positive line on a COVID test, and what it means for the 10 days after your positive result.

What Line Intensity Might Say About a Person's COVID Infection

The reason that at-home tests display lines that look so different from one another comes down to how these tests are formulated, Dr. Chin-Hong explained. As your sample saturates the test strip, it interacts with antibodies that release a color as they bind to the virus.

You can think of a test strip as a venus fly trap, and the virus particles as the flies, Dr. Chin-Hong said. "The more [the] Venus fly traps snap, the higher the color because the dye is really linked to that reaction of a match," he said.

The same thing is true for the speed at which your test shows a positive line: If there are higher loads of virus in the body, the color on the test will show up much more quickly. A fainter line will probably take longer to show up because it's taking more time for the antibodies to match with pieces of the virus.

This is also why a negative antigen test after five days of isolation likely means that the person is no longer contagious, Dr. Chin-Hong added. The virus levels in their body are gone or are so low that, after masking for an additional five days, they should be in the clear.

Because of this, we might expect for a fainter line—and less virus in the body—to indicate that a person is less sick, less infectious, or further along in their infection. And sometimes, this is a fair assumption to make, said Eric Vail, MD, director of molecular pathology at Cedars-Sinai.

"Usually asymptomatic people tend to have lower levels," Dr. Vail told Health. "They do have smaller amounts of virus generally in their nose and so they tend to have lighter lines."

Also, if people test multiple times over the span of their infection and see the lines growing fainter, that's probably a good sign that they're almost done with their infection, Dr. Chin-Hong added.

However, at-home tests have a ton of variables. Just because a faint line might mean the impending end of an infection for one person, it just may mean user error—like if a person didn't swab their nose correctly—for another.

"If there was some universal sampling mechanism, and everybody got sick in the exact same way at the exact same time and you tested at the exact same time, then maybe we could start to draw some conclusions from that," Dr. Vail said.

But, with a lack of research on the subject and the range of variability in time and strategy with which people test themselves at-home, changing safety precautions depending on the faintness or darkness of a line is not recommended.

A Positive is a Positive, No Matter How Faint the Line

Because the line intensity of a positive COVID test can be dependent on so many factors, experts say it's best to stick with the guidelines we have in place: That any line on an antigen test—whether it's faint or dark—is considered a positive test, and that a person should go into isolation.

"It's one of the things that intuitively, it works and it makes sense, but I just would caution [against] trying to use it as a surrogate to following the guidelines," Dr. Vail urged. "Five days of [isolation] and then five days of masking if you have no symptoms, and just kind of go from there."

The way that at-home tests currently work, Dr. Vail explained, is to give the person a yes or no response to the question of whether or not they have the virus in their bodies. Any information we might glean otherwise—how infectious or symptomatic we think we are or might be—is just a guess.

This is even true for people who continue to test positive after the Centers for Disease Control and Prevention's (CDC) recommended five-day isolation, Dr. Vail explained.

"Say [on] day six you want to do a test to see if you're positive. Well, day six, about 50% of people are still positive on the antigen test. Does that mean they're still infectious? We don't know," Dr. Vail said.

Right now, it's not possible to tell whether that virus being detected is active and making you infectious, or if it's just dead virus being incorrectly pinged by the test. But, according to CDC guidance, that additional positive test gets you five more days of isolation.

At-home tests have an extremely low chance of giving you a false positive, so regardless of whether it seems like you have a lot of virus or just a little, if the test is picking up on it, the best thing is to follow antigen test kit instructions and CDC guidelines, and seek treatment if you think you might need it.

"These things that people are observing are just like little signals," Dr. Chin-Hong said. "Say you're at home and your line is getting fainter and fainter and fainter. Well, maybe you might feel better about coming out of your room to see your family, if you wear a mask. I think it might be helpful in little environments like that, but we can't use [line intensity] for the large public."

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