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

A few weeks ago, after dodging COVID-19 for more than two years, I tested positive on a rapid home test. The line was barely there – so faint it couldn’t even be seen in photos. Have I cheated myself? Unfortunately, not. The much more obviously positive test I took the next day confirmed 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 anything about your individual infection.

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

Quite literally, the positive line of a rapid at-home test “shows the presence of targeted viral proteins,” Omai Garner, Ph.D., associate clinical professor and director of clinical microbiology at UCLA Health, told TODAY.

“It’s looking for a specific part of the virus that attaches itself to components of the test that are bound to a color,” said Dr. Emily Volk, President of the College of American Pathologists, TODAY.

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

If this positive line appears, it’s 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, said the experts.

“It’s not a super-sensitive test, which means you have to have a good amount of virus in there just for the home antigen test to even work,” Garner said. Keep that in mind”any Line early in the infection process implies someone is highly contagious.”

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

It can also be helpful to consider the context of what’s going on around you. If COVID-19 transmission rates 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 any noticeable symptoms, these are all good reasons , to interpret a perhaps-positive as definitely-positive.

“Particularly with the amount of COVID that is circulating, it should be taken as a positive until proven otherwise,” Mathers said.

If you want to confirm the result, you can do another rapid test a day or two later. If your second test is also pretty faint or you don’t have a line at all, that’s a good time to do 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 straight to their doctor or a PCR test.

The only time you wouldn’t assume a faint line on a rapid test is positive is if it becomes positive after the allotted testing time, Garner said. “If you’ve only left the test for two hours, you can have a false positive bond,” he 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 super dark?

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

But those tests weren’t really designed to measure any of that, the experts said. “These antigen tests are qualitative, so they’re not designed to give you an assessment of, ‘Is there a lot of virus or a little virus?'” Volk explained.

They should really only be read as 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. Plus, there are a number of other reasons a test line might be darker or lighter that have nothing to do with the actual amount of virus particles in your body, she said.

For example, the consistency of your snot can affect how many of these viral proteins accumulate in your nose. “So you could have a lot of viral antigen in your nose,” but that may not accurately reflect how much virus is actually circulating in your system because your mucus is just extra thick, Mathers explained. (Snot, like saliva, can be thicker or thinner depending on how hydrated you are, she said.)

Additionally, the pH of your nasal ecosystem “could change how well the virus binds,” she said. “All of these variables in human samples can change the way the test can be read.”

The room temperature when the test is performed, as well as how the tests are stored, can also affect the results, as TODAY previously explained.

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

Given the convenience and availability of rapid tests, it’s understandable that people would want to use them in ways that aren’t necessarily intended, Garner said. “People try to use the antigen tests not only to help diagnose diseases, but also to help their behavior after an infection.” This is especially true in sensitive situations where people are pressured to come back to the Going to work as soon as possible or having to make difficult decisions about participating in other activities — even if they still test positive, he said.

But you shouldn’t use the lightness or darkness of the line on your test to guide your behavior because the tests just aren’t designed or FDA approved for that, Volk and Mathers agreed.

For example, if your line is lighter, that doesn’t mean you can ignore other precautions like covering it up. “There’s really no actionable information (when you look 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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