At-home rapid COVID-19 tests can reveal more about viral load than a simple positive/negative result, according to experts.
“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,” Michael Mina, MD, PhD, chief science officer for eMed and a well-known public health expert who shares his expertise and views on the social media site known as X and in numerous publications, told MedPage Today.
Mina recently shared a schematic of rapid COVID test results and the potential implications of their variability. Mina has been a vocal proponent of this testing technology, so people can better understand their own infection, and he has not been alone in highlighting these ideas. Researchers have shown that variations in COVID test results can reveal different aspects about an individual’s infection and, critically, how contagious they are at a given moment.
A 2021 study showed that rapid antigen tests were sensitive and specific enough to detect a person’s viral load, whether they were symptomatic at the time or not. The authors noted that their study 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.”
Other studies also showed that rapid antigen tests were capable of showing more detail about viral load than has been widely promoted.
Jeremy Faust, MD, editor-in-chief of MedPage Today, also wrote about the nuances of rapid antigen test results, emphasizing that a major problem with these tests is “they often don’t mean what people think.”
While a positive test result means the person has COVID, negative tests are not as conclusive, he noted.
How to Decipher a Rapid COVID Test
Mina said the first thing to consider about the rapid antigen tests is that they are built like virus traps. To show whether a person has enough virus to be considered contagious, the tests are designed to catch all the virus available in a test sample, which then appears on the test line.
Another important fact about the tests, he added, is that they do not include an amplifying step, which means the virus captured on the test line is a true representation of an individual’s viral load.
“It’s very intuitive when you start thinking about it,” he 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.”
This means that people can interpret their test results in two simple ways: time to test line appearance and test line darkness after 15 minutes, he explained.
The basics of these variations come down to showing how much viral load a person has at one point in time. A darker test line or a faster test line mean a higher viral load. Mina noted that a person with a dark line that appears quickly has a very high viral load, and he or she is likely near the peak of their infectiousness. Conversely, a lighter line that appears closer to the 10- to 15-minute mark means viral load is low, he added.
In addition to measuring viral load at one specific time, Mina said that a series of tests could be used over a few days to determine where a person is in their infection. For example, if someone has a lighter test line on day 1 and a darker test line on day 3, then they are entering into their most contagious phase of their infection.
If a person used this information correctly, they could understand their infection better than just monitoring their symptoms, which are not necessarily correlated with their viral load, Mina explained. This would help an asymptomatic person know how long to isolate, and it could also help someone who continues to see a dark test line to understand that they need more medical attention to address their infection.
He emphasized that if an individual is in doubt about the meaning of their test results, then the simplest approach is to consider any positive test line as a sign of contagiousness.
“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.”