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Is a faint line on the COVID-19 test really positive? Experts explain how to interpret the results

A few weeks ago, after more than two years of avoiding COVID-19, I tested positive on a rapid home test. The line was barely there, so faint that it didn’t even show up in the photos. Was I kidding myself? Unfortunately not. The much more obviously positive test I took the next day confirmed that I had COVID-19.

The whole experience also got me thinking about what the line actually means and whether a darker or lighter positive line on a COVID-19 test can tell you something about your individual infection.

What does the line actually measure in a COVID-19?

At its most literal level, the positive line on a rapid home test “shows the presence of specific viral proteins,” Omai Garner, Ph.D., clinical associate professor and director of clinical microbiology at UCLA Health, told TODAY.

“You’re looking for a particular part of the virus that sticks to test components that bind to a color,” Dr. Emily Volk, president of the College of American Pathologists, told TODAY.

From there, the proteins “get caught on that line and show a band of color,” Dr. Amy Mathers, an associate professor of medicine and pathology and associate director of clinical microbiology at the University School of Medicine, told TODAY. of Virginia.

If that positive line appears, it is very likely that you have coronavirus proteins in your nose, and that you have COVID-19.

Does a faint line count as a positive result?

Yes, the experts said.

“It’s not a super sensitive test, which means you have to have a fair amount of virus in there just for the home antigen test to work,” Garner said. Bearing that in mind, “none early line in the infectious process implies that someone is highly contagious.

But that doesn’t mean it’s always easy to read. “Sometimes it’s not exactly a line; it can be like fluff,” Mathers said. “But if you see a line there, it’s there.”

It can also be helpful to take into account the context of what is happening around you. If COVID-19 transmission levels are high in your area (as they are in much of the country right now), if you know you’ve been exposed to someone with the infection, or if you have obvious symptoms, those are all good reasons to interpret a maybe-positive as definitely-positive.

“Especially with the amount of COVID circulating, it should be considered positive until proven otherwise,” Mathers said.

If you want to confirm the result, you can take another quick test a day or two later. If your second test is also pretty weak or doesn’t have any lines at all, that’s a good time to get a PCR test to see what’s really going on, Garner said. Both Volk and Mathers suggest that people who are unsure of their results skip the second rapid test and go directly to their doctor or have a PCR test.

The only situation in which you wouldn’t assume a faint line on a rapid test is positive is if it turns positive after the allotted testing period, Garner said. “If you left the test for two hours, you can have a false-positive junction,” she explained. “But if the test is done correctly, any line, no matter how faint, is a true positive.”

Does it matter if the line on your COVID-19 test is very dark?

In theory, “the more viral proteins there are, the darker the line will be,” Garner said. And from there, you might conclude that you are more or less contagious or that you might have a milder or more severe infection depending on how dark or faint your line is.

But these tests weren’t actually designed to measure any of that, experts said. “These antigen tests are qualitative, so they’re not designed to give an estimate of ‘Is there a lot of virus or is there a little virus?’” Volk explained.

They are really meant to be read as a binary: positive or negative.

“We have some of these tests in our lab that we run as medical tests, and we don’t interpret the strength of the (line) at all,” Mathers added. In addition, there are many other reasons a test line might be darker or lighter that have nothing to do with the actual amount of viral particles in your body, he said.

For example, the consistency of the mucus can affect the amount of viral proteins that accumulate in the nose. “So you may have a viral antigen load in your nose,” but that may not be an accurate reflection of how much virus is actually circulating in your system because your mucus is so thick, Mathers explained. (Mucus, like saliva, can be thicker or thinner depending on how hydrated you are, he said.)

Also, the pH of your nasal ecosystem “could change how well the virus binds,” he said. “All of those variables in human specimens can alter how the test reads.”

The ambient temperature when you are running the test, as well as how well the tests are stored can also affect the results, as explained earlier in TODAY.

We know that early in infection, people can be very contagious and have a lighter line on their rapid antigen test, or not be positive at all. They may even have symptoms for a few days before turning positive. “People can have severe COVID infections and a faint line, and people can have mild COVID infections and a very deep red line,” Volk said.

With the convenience and availability of rapid tests, it’s understandable that people want to use them in ways that aren’t necessarily intended, Garner said. “People are trying to use antigen tests not only to help diagnose the disease, but also to help their behavior after they’ve been infected.” That is especially true in those difficult situations where people may be pressured to return to work. As soon as possible or they have to make tough decisions about participating in other activities, even if they still test positive, she said.

But you shouldn’t use the lightness or darkness of the line in your test to direct its behavior because the tests simply aren’t designed or FDA-cleared for that, Volk and Mathers agreed.

If your line is lighter, for example, that doesn’t mean you can ignore other precautions, like masking. “You can’t really get actionable information (by looking at whether your line is lighter or darker),” Volk explained. If it’s positive, it’s positive, and you can probably leave it at that.

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