What does a faint line on a rapid COVID-19 test mean?

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As convenient as it may be to take a COVID-19 test at home, the results from those easy-to-use rapid antigen tests can also be confusing. If you get a very faint line, for instance, it might be hard to know whether or not that really means you're positive.

It even happened to me: After more than two years of evading COVID-19, I tested positive at home on a rapid antigen test during the summer of 2022. At first, the line was so faint that it didn't even show up in photos. Looking at the faint line, I wondered if I was just tricking myself or if I was actually positive for COVID-19.

When I took another test the next day, however, the line came back much darker than the day before. And that test confirmed that I had COVID-19.

The whole experience got me thinking about how confusing it can be to take a rapid antigen test at home — especially if you get a faint positive line like I did. With seasonal respiratory illnesses — including flu, RSV and COVID — all circulating at high levels right now, it’s especially important to understand how to use COVID-19 tests properly.

"The at-home kits are great," Dr. Diana Cardona, a member of the College of American Pathologists Board of Governors, tells TODAY.com. "I use them for my family because (they're) easy," she says.

"But the key is really understanding (their) limitations," adds Cardona, who is also the vice chair and director at Duke Health Anatomic Pathology Laboratories.

First, it's important to read the instructions before taking the test because different brands can be slightly different from each other, she says. From there, you should know what your results actually mean, including a faint positive line, and when to take another test or get a more sensitive COVID-19 test from your doctor to clear things up.

"Getting tested lets you know what you have, which can ensure you get the treatment that could save your life," Dr. Mandy Cohen, director of the Centers for Disease Control and Prevention, said on a press call on Dec. 1, 2023. "Right now, respiratory illness is spreading in most of the country and we do expect COVID, flu and RSV levels to continue to increase," she said.

When it's time to take a test, experts want you to know what the line on your test actually means and whether a darker or lighter positive line on a COVID-19 test can tell you anything about your individual infection.

Do rapid antigen tests still detect new coronavirus variants?

The landscape of coronavirus variants has changed a lot since the pandemic first began.

As of late November, the dominant strains are all subvariants of the omicron variant, according to the most recent estimates from the CDC. The most common strains right now include HV.1, which is responsible for nearly 32% of cases in the U.S., and EG.5 (also called Eris), which is causing about 13% of cases, CDC estimates show.

Luckily, at-home rapid antigen tests will still be able to detect coronavirus variants that emerged more recently, Cardona says. "We have not seen any data to suggest (that the tests don't work on new variants) yet," she says.

That's because the antigen that these tests detect doesn't really change from strain to strain, Cardona explains.

When to take a rapid COVID-19 test:

You should take a COVID-19 test at home in a few specific scenarios, the CDC explains. First, if you have noticeable symptoms that could be related to COVID-19, you should take a test.

These are some of the most common symptoms of COVID-19, according to the CDC:

  • Fever

  • Chills

  • Sore throat

  • Loss of taste or smell

  • Runny nose or congestion

  • Shortness of breath

  • Fatigue

  • Muscle or body aches

  • Nausea

  • Vomiting

  • Diarrhea

You should also take a test if you're exposed to someone who has COVID-19, the CDC says. To avoid testing too early in an infection, the CDC recommends testing five days after the exposure. In the event your results are negative, the Food and Drug Administration recommends testing again in 48 hours and, possibly, a third test another 48 hours later.

"If you know you had a positive exposure — with or without symptoms — and you get a negative result, I would probably be a little skeptical of that," Cardona says. "That might be a false negative result," she explains, which means you should take another test a day or two later.

Finally, consider testing for COVID-19 before attending an indoor gathering or visiting someone. Try to take the test as close to the start time of the event as you can in order to get the most accurate results possible, the CDC advises.

It's especially important to test before gathering indoors with people who are at a higher risk for severe COVID-19 symptoms, such as older adults, people who are immunocompromised and children who are too young to be vaccinated, the CDC says.

How to take an at-home COVID-19 test:

When using a home rapid test, it's important to avoid some common mistakes, experts tell TODAY.com.

First, be sure to store the tests properly (and at the right temperature). If rapid antigen tests are stored in an environment that's too cold or too hot, they may give inaccurate results, including false negatives.

And when you're ready to take the test, read the instructions fully before starting. Tests from different brands may be similar, but they're not identical. So it's important to know what you're doing before you get into it.

Don't read the test too early or too late, the experts say, because that may give you a false-negative or false-positive result. Only read your results within the time window that the COVID-19 test instructions give you.

And, depending on your results and your exposure history, you may need to repeat the test again in 48 hours, the FDA says.

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

At its most basic level, the positive line on an at-home rapid test "is showing the presence of targeted viral proteins," Omai Garner, Ph.D., associate clinical professor and director of clinical microbiology at UCLA Health, tells TODAY.com.

"It's looking for a particular part of the virus that attaches to components of the test that are attached to a color," Dr. Emily Volk, past president of the College of American Pathologists, tells TODAY.com.

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

If that positive line shows up, 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, the experts say.

"It's not a super-sensitive test, meaning you've got to have a good amount of virus there just to get the home antigen test to work at all," Garner says. Keep in mind that "any line early in the infectious process implies that somebody is very 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 says. "But if you see a line there, it's there."

It can also help to take in the context of what's going on around you. If COVID-19 transmission levels are high in your area, if you know you were exposed to someone with the infection or if you have noticeable symptoms, those are all good reasons to interpret a maybe-positive as a definitely-positive.

In order to confirm the result, the FDA recommends taking another rapid test 48 hours later. If you have symptoms that could be COVID-19 or you know you were exposed to COVID-19 recently, the FDA recommends taking yet another test 48 hours later.

If there's any confusion throughout the process, you can also skip the repeated rapid tests and go straight to your doctor or get a PCR test, the FDA says. However, the CDC notes that people who've had COVID-19 may continue to test positive on PCR tests for up to 90 days, so it may be difficult to use a PCR test to diagnose a new coronavirus infection.

It is possible to have a COVID rebound after testing negative, Cardona says. This occurs when someone seemingly recovers from a COVID-19 infection, tests negative and then tests positive again just a few days later. It can happen with or without taking antiviral medications, such as Paxlovid.

If you get a rebound positive result, you may or may not have symptoms, Cardona says, and it's not clear how contagious people are in the rebound phase, experts told TODAY.com previously. But it's best to err on the side of caution by distancing yourself when possible and wearing a mask when you're around others.

The only situation in which you wouldn't assume that a faint line on a rapid test is positive is if it turned positive after the allotted testing period, Garner says. "If you just left the test for two hours, you can have some false positive binding," he explains. "But if the test is performed correctly, any line — no matter how faint — is a true positive."

Also, keep an eye on the expiration date on the rapid tests you're using. If you use a home test after its expiration date, you may not get accurate results. The FDA has information about the shelf life and expiration dates — including some that have been extended by several months — for all of the home tests that it has authorized.

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

In theory, “the more viral proteins that are there, the darker the line would be,” Garner says. And, from there, you might conclude that you’re 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 the experts agree that these tests weren’t really designed to measure any of that. “These antigen tests are ... not designed to give you an estimate of, ‘Is there a lot of virus or is there a little virus?’” Volk says.

They're really just 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 do not interpret the strength of the (line) at all,” Mathers adds. Plus, there are a bunch of other reasons a test line might be darker or lighter that don’t have anything to do with the actual amount of viral particles in your body, she says.

For instance, the consistency of the mucus in your nose may affect how many of those viral proteins you collect. “So you might have a load of viral antigen 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 just extra thick, Mathers explains. (Mucus, like saliva, can be thicker or thinner depending on how hydrated you are, she says.)

Additionally, the pH of your nasal ecosystem “could change how well the virus binds,” she says. "All of those variables in human specimens can alter the way the test may read."

The room temperature when you're running the test as well as how well the tests are stored can also affect the results.

We know that early on in the infection, people tend to be more infectious yet may have a lighter line on their rapid antigen test — or not test positive at all. They may 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 says.

With the convenience and availability of rapid tests, it's understandable that people will want to use them in ways they aren't necessarily intended, Garner says. “People are trying to use the antigen tests not only to assist in the diagnosis of disease but also to assist in their behavior after they’ve been infected," he says.

That's especially true in those tricky situations where people may be pressured to get back to work or have to make tough decisions about partaking in other activities — even if they're still testing positive late into an infection, he says.

But you should not use the lightness or darkness of the line on your test to direct your behavior because the tests are simply not designed or FDA-authorized for that, Cardona and Mathers agree.

If your line is lighter, for instance, that doesn't mean you can ignore other precautions, like masking. “There’s really no actionable information to be gained (from looking at whether your line is lighter or darker),” Volk explains.

If it's positive, it's positive — and you can probably leave it at that.

This article was originally published on TODAY.com