What rapid COVID-19 tests can reveal about your viral loads

According to health experts, rapid COVID-19 tests can provide patients with more information about their current viral load than just a simple positive or negative result.

"By definition, the basic technology suggests that you somehow have to go from a negative 'zero' line to a dark line, and within that window -- that 0 to 100% -- there's obviously a gradient," said Michael Mina, a public health expert and chief science officer for eMed.

Researchers have also found that rapid COVID-19 tests can reveal different information about people's infections and how contagious they are at a certain point in time depending on the variations in their results.

For example, a 2021 study found that rapid COVID-19 tests were sensitive and specific enough to detect someone's viral load, even if they were not symptomatic at that point. The results "indicate a clear relationship between relative viral load and test positivity and provide a practical, real-world criterion to assist calling results in this setting," the authors wrote.

How to interpret a rapid COVID-19 test

According to Mina, when a person takes a rapid COVID-19 tests, they should pay attention to the time it takes a positive test line to appear and how dark the line is after 15 minutes.

If a positive test line appears quickly or is very dark, then a person has a higher viral load. Mina also noted that a dark test line that appears quickly suggests a very high viral load, and the person is likely near their peak infectiousness. In comparison, a lighter line that appears around 10 to 15 minutes later means that the person's viral load is likely low. 

"It's very intuitive when you start thinking about it," Mina said. "If you see a line right away, I think a lot of people just intuit for themselves: 'Wow, I have a lot of virus in my nose.' If it takes 15 minutes for you to start to see a line ... then it makes you realize you probably don't have a lot of virus, and everything in between is that gradient."

Because rapid COVID-19 tests only measure viral load at one specific point in time, Mina recommends people test themselves multiple times over a few days to see where they are in their infection. A lighter test line on day one could become darker on day 3, which suggests an increase in viral load and contagiousness.

These results could help people understand their infection better than just monitoring symptoms, which are not necessarily correlated to viral load, Mina said. For example, an asymptomatic individual could know how long they need to isolate, or someone with a dark test line would know that they could need medical attention for their infection.

"These tests are really good indicators of how infectious you are," Mina said. "There's so much anxiety around having this virus that I think it helps people to know what's happening inside their body. There is some real public health utility to it."

Separately, Jeremy Faust, an emergency medicine physician at Brigham and Women's Hospital and MedPage Today's editor-in-chief, noted that while a positive COVID-19 tests means you have an infection and are likely contagious, a negative test doesn't necessarily mean that you're not infected.

Depending on when you test yourself, a negative COVID-19 test can mean different things:

• It's too early in your infection for a rapid test to detect your viral loads, and you could test positive later
• It's too late for a rapid test to detect viral loads because you've been sick with COVID-19 for a while and are no longer contagious
• You don't have COVID-19 if you test negative on a test for six to seven days after a high-risk exposure

"Everything would be perfect if people could remember the following about rapid antigen tests: Negative? You might have COVID but you're not contagious now. Test again at least a couple more times over the coming days. Positive? You're infected and contagious," Faust writes. (DePeau-Wilson, MedPage Today, 10/6; Faust, MedPage Today, 9/28; Mina, X, 9/26)

The development of COVID-19 tests and reliance on home-based care have made at-home testing a major market — but is it here to stay? We examine two factors driving its growth and two potential roadblocks to its continued success.