Claire Wolters is a Philly-based reporter covering health news for Verywell.

Heather Mercer is native to Northwest Ohio (go Walleye!) and graduated from Loma Linda University with two doctorate degrees (psychology and public health). She is currently a professor at Owens Community College, as well as a fact-checker for Verywell Health. She has gained experience in a variety of settings, ranging from corporate wellness and preventive medicine, to mental health, chronic disease, and end-of-life care. 

Verywell Health / Photo Illustration by Amelia Manley / Getty Images

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 are indicative of how sick you are, and how likely you are to transmit COVID-19 to others.

Michael Mina, MD, PhD, chief medical officer at eMed and former Harvard epidemiology professor, told Verywell the darkness of a line on a positive rapid test is reflective of how much virus a person has, which is the best indicator for their level of infection.

“If your line is really dark, you might be a super spreader; you might go into a bar and infect 30 people,” Mina said. “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.”

A spokesperson for Abbott, which produces the widely available BinaxNOW COVID-19 Antigen Self Test, told Verywell that it is most important for people to know that if a second line shows up in addition to the control line, to any extent, it means that the test is positive and the person has potential to transmit the virus.

Mina said a test that turns positive quickly, such as in a matter of seconds, indicates a high viral load and high potential for transmission.

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

Mina emphasized a slow-appearing faint test can still represent high levels of virus.

"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," he said.

With this in mind, Mina said it's important for people to continue to wear a mask and be mindful of social interactions, particularly those that involve vulnerable groups, even if their line is almost non-existent.

"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 infections if you previously had a really dark line, and now your line has become really faint."

But even with a faint line on a positive test, Mina added: "Do not go visit Grandma."

If you believe you're at the start of a COVID-19 infection and see a faint line on your test, treat even more carefully, Mina said, since viral load can increase in a matter of hours or days.

Rapid COVID tests are better a detecting real-time infectiousness than PCR tests. Knowing that you have a high viral load—as indicated by a dark line—should encourage you to steer clear of others for the time being, since you are highly contagious.

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

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

Both rapid and PCR tests have the potential to be wrong. But Mina said it is highly unlikely that a person would test positive on a rapid test if they do not have COVID-19, meaning a false positive is rare.

After conducting a study on vaccinated college students, Tara Bouton, MD, MPH, attending physician in the Section of Infectious Diseases at Boston Medical Center, thinks false positives are plausible at the tail end of an infection.

In her study, about 50% of students who tested positive on a rapid test five days after symptom onset were negative in the lab culture tests, leading the researchers to believe that the rapid tests had, in some cases, detected a residual protein—not an active virus.

"Based on what we've seen around that five-day mark, [negative test requirements] are keeping a significant number of people out of activities that potentially no longer have a replicating, potentially transmissible virus,” Bouton said. "Still, the most important thing to stress is the ongoing strict mask-wearing for the additional five days, because it is so difficult to interpret whether or not somebody still has transmissible virus or not."

Bouton said her study, which has yet to be peer reviewed, had sampling limitations, including the fact it evaluated a largely young and healthy cohort of vaccinated college students. If verified on a larger scale, however, these results could suggest that testing alone is keeping some people out of daily activities they could otherwise partake in.

If your rapid COVID test is positive, you should believe it. The darker the line, the more infectious you are, and the more important it is to wear a mask and avoid others. But even faint lines can indicate the presence of infection.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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Advisory Board. Some people test positive for Covid-19 for weeks. Are they still infectious?

Bouton T, Atarere J, Turcinovic J, et al. Viral dynamics of Omicron and Delta SARS-CoV-2 variants with implications for timing of release from isolation: A longitudinal cohort study. Infectious Diseases (except HIV/AIDS); 2022. doi:10.1101%2F2022.04.04.22273429

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