What Does the Darkness of Your COVID Rapid Test Mean?

Does a strong, dark line on a COVID-19 rapid test mean you're more infectious than a faint line? 

According to experts, both the darkness of the line and how long it takes to appear can show how much of the COVID virus is in your body and how likely you are to spread it to others—but there are also some limitations you should be aware of.

Here’s what you need to know about how to interpret the lines on your COVID test and what to do if the line is faint.

What Lines on COVID Tests Mean

When you open up an at-home COVID test, you might see a line on it already—that’s the control line. When you do the test, you’re looking to see if another one shows up. For some at-home tests, the control line only appears after you've added your sample to the test, so be sure to check the instructions to know when the control should be visible.

How that line appears will vary based on how much virus there is, how you did the test, whether the test has expired, and even the brand of COVID test you happen to be using.

Does the Darkness of the Line on a COVID Test Matter?

As researchers have learned more about how at-home rapid tests work in the real world, they’ve found that in a lot of cases, how dark the line looks on the test does line up with how much virus is in a person’s body. 

The “viral load,” in turn, often lines up with how sick a person is and how likely it is they can infect other people. 

Michael Mina, MD, PhD, chief medical officer at eMed and a former epidemiology professor at Harvard, told Verywell that the darkness of a line on a positive rapid COVID test shows how much virus a person has in their body. How much virus is in your body can also give you a sense of how likely it is you'll give the virus to someone else.

“If your line is really dark, you might be a super spreader; you might go into a bar and infect 30 people,” said Mina. “If your line is really light, especially on the back end of the infection, you could infect your spouse, who you're sleeping head to head with on a pillow…but you're probably not going to walk into a room and mistakenly infect 30 people.”

Other experts point out that variations between brands of tests and factors like user error mean you probably should not rely on the darkness of a test line to predict how sick a person will get or how many people they could spread the virus to.

For example, you could still get really sick with COVID even if the line on your test wasn't that dark—especially if you have risk factors like underlying health conditions.

Where experts do tend to agree is that it's best to be cautious and proactive if you get a potentially positive COVID test.

A spokesperson for Abbott—the company that makes one of the most widely available at-home rapid COVID tests (BinaxNOW COVID-19 Antigen Self Test)—told Verywell that the most important thing to know is that if that second line shows up on your test, even if it's faint, it's positive.

If you get a positive test, you have to operate under the assumption that you have COVID and could spread it to other people, even if you don't feel sick.

What Does It Mean if Your Test Turns Positive Quickly?

Mina said a COVID test that turns positive in a matter of seconds shows you have a high viral load—and a higher potential to spread the virus to others.

"If a line gets to its peak darkness after 10 minutes, that means you have a lot of virus," said Mina. "But if it gets to its peak darkness after 20 seconds, then that means you have a tremendous amount of virus.”

Are Rapid Tests Better Than PCRs?

Rapid COVID antigen tests will better detect real-time infectiousness than PCR tests, but they both have uses in specific situations. 

"An antigen test is sort of like a security guard," said Mina, explaining that rapid tests detect viral proteins. "A security guard works by recognizing that something bad is happening right at that moment, and is able to act immediately." 

Mina said that a PCR test detects a virus's genetic material and is more like a forensics detective, who "shows up at the scene of the crime after the crime has already taken place.”

What to Do If You Get a Faint Line

If you get a quick, dark line, it’s probably going to be pretty clear to you that your COVID test is positive. However, Mina emphasized a slow-appearing faint line on a test or even one that’s sort of blurry or fuzzy can still mean you have high levels of virus in your body. 

"For a virus to gather enough molecules on these tests to actually be able to show a line means there is a really large amount of virus there," said Mina.

If you think you're at the start of a COVID infection and see a faint line on your test, Mina said that you need to be more careful since your viral load can increase in a matter of hours or days. 

On the other side of infection, if you’re still seeing a faint line even after your symptoms have gotten better or you’ve been treated for COVID (for example, with Paxlovid), it’s better to take it as a sign that you could still be contagious.

While that may not be the case because some people may keep testing positive even when they’re no longer capable of spreading the virus, you can’t know for sure.

To make things even more confusing, if you’ve had COVID before or have been vaccinated/boosted, you could have a faint line at first, and then negative tests if your body is able to fend off the virus quickly. However, it's important to note that being vaccinated will not make you test positive unless you're using a test to check for antibodies and not an active infection.

Getting some clarity on a faint line is where serial testing to try to “catch” peak viral load can come in handy. If you get a negative or a questionable positive on your first test, retesting in a day or two may give you a much clearer result.

Mina said that even if the line is almost non-existent, the safest route is to assume you’re still positive and take steps to prevent infecting others.

If you test positive for COVID, the Centers for Disease Control and Prevention (CDC) recommends that you stay home and away from others until you've been fever-free for 24 hours and symptoms are improving. Taking additional precautions for the next five days is also encouraged. This includes wearing a mask, improving ventilation, washing your hands often, and keeping a distance from others.

"It's a good rule of thumb to assume that you're infectious if you're positive at all," Mina said. "I also think it's a reasonable thing to try to explain to people that you're less infectious if you previously had a really dark line, and now your line has become really faint.”

That said, even with a faint line on a positive test, Mina advised: "Do not go visit Grandma."