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

If you're testing for COVID-19, you'll want to know what a positive test looks like—and what it means if you get a faint line. For those who 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. The test is positive even if only a faint line appears.

At-home COVID 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 confusing, especially with so many factors at play.

What COVID Test Line Intensity Means

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 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, Eric Vail, MD, director of molecular pathology at Cedars-Sinai, told Health. So, 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, keep in mind, this isn't the case for everyone.

No COVID test can tell you for sure how long your illness will last and 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
  
• 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
• The impending end of infection

Rapid tests can break down over time, making them less sensitive and less reliable. This degradation 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 Line Intensity 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, said Dr. Chin-Hong. As your sample saturates the test strip, it interacts with antibodies that release a color as they bind to the virus.

The 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.

A Faint Line Is a Positive Test

Essentially, a positive result is still positive, no matter how it looks. Line intensity isn't a perfect measure of how infectious or sick you might be. However, a faint line may indicate you do not have as strong of a viral load in your body as a darker line might. This may be because you have a milder infection or are on the tail end of your illness. Regardless, if you see a line, no matter how faint, it means you are positive for COVID.

The way at-home tests currently work, said Dr. Vail, is to give the person a yes or no response to whether 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, explained Dr. Vail.

This is also true for people who continue to test positive even after they no longer have a fever and are feeling better. 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, any positive test means you may still be contagious.

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.

When To See a Healthcare Provider

Contact a healthcare provider if you are unsure about your COVID test results or if you have any other COVID-related questions. They can help you interpret your results and let you know if other testing should be done.

A Quick Review

Any line on a COVID-19 antigen test indicates a positive test, regardless of its intensity. This means faint lines still indicate a positive test. 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 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. If you have any questions about your COVID test or symptoms, contact a healthcare provider.