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.

Fact checked by Marley HallMedically reviewed by Kashif J. Piracha, MD

At-home COVID-19 tests provide a relatively simple and accessible way to see if you've become sick with the virus. However, 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. Many experts agree that you should consider the test positive even if only a faint line appears.

Here's what to 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.

Related: How Well Do Rapid COVID Tests Detect Omicron and Its Subvariants?

What Line Intensity Might Say About COVID

The darkness of a positive line on a COVID test is related to how much virus is in your body. "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."

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 might indicate that you're less sick, less infectious, or further along in your infection. Sometimes, this is a fair assumption to make, said Eric Vail, MD, director of molecular pathology at Cedars-Sinai. Therefore, if people test multiple times throughout their infection and see the lines growing fainter, that's probably a good sign that they're almost done with their infection.

However, no COVID test can tell you for sure whether or not you're contagious or can spread the virus to others.

At-home tests also have numerous variables, so a faint line may indicate:

• An expired test, which might not be as accurate

• The impending end of infection

• Lack of symptoms, because tests aren't always as sensitive if you're asymptomatic

• User error—for example, you didn't swab your nose correctly

Rapid tests can break down over time, making the test less sensitive and less reliable. The degradation of these tests is why results from expired antigen tests shouldn't be wholly trusted.

What if the Control Line Is Faint?

Does the intensity of the control line make a difference in diagnosis? Not really.

If you can see the control line, no matter how faint, the test is valid. If you can't see the control line, the test is invalid, and you'll need to use another test.

Does It Vary by Test Type?

The reason that at-home COVID 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.

The length of time it takes for a test to detect the virus also depends on the type of test you use, as well as the COVID-19 variant.

The FDA generally recommends repeat testing if you initially test negative regardless of the type of antigen test you use to reduce your risk of a false negative result. This applies whether or not you have symptoms.

Related: When Should You Test (and Possibly Retest) After Being Exposed to COVID-19?

A Faint Line Is a Positive Test

Line intensity isn't a perfect measure of how infectious or sick you might be. Essentially, a positive result is still a positive result, no matter how it looks.

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 also true for people who continue to test positive after the Centers for Disease Control and Prevention's (CDC) recommended five-day isolation. About 50% of people test positive using an antigen test 5-9 days after symptoms begin. A positive test doesn't necessarily mean you're contagious—and a negative test doesn't mean you're not.

At this point, it's not possible to tell whether the virus being detected is active and making you infectious, or if it's just a dead virus being incorrectly pinged by the test. However, according to CDC guidance, that additional positive test means you need to continue to wear the mask until day 10.

At-home tests have an extremely low probability of giving you a false positive. Whether or not it seems like you have a lot of virus or just a little, follow antigen test kit instructions and CDC guidelines, and seek treatment if you think you might need it.

A Quick Review

Any line on a COVID-19 antigen test indicates a positive test, regardless of its intensity. Factors that might influence the line's appearance include your viral load, whether or not you have symptoms, and not using the test kit correctly. A fainter line might indicate that you're less contagious, but no test can definitely tell you whether or not you're contagious.

Changing safety precautions depending on the faintness or darkness of a line is not recommended due to the range of variability in time and strategies with which people test themselves at home.

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