• Posted June 26, 2024

Feel Sick? Waiting at Least 2 Days Before COVID Test Is Best

The COVID virus, or rather people's immune response to it, has changed and it might be prudent to wait a couple days after symptoms start before taking a COVID test, researchers report.

“For COVID, we found that if you only have one test, it’s best to wait two days after symptoms arise to use it, because the virus is unlikely to be detectable until then,” said study first author Casey Middleton, a PhD student in the department of computer science at the University of Colorado Boulder (UCB).

“For flu and RSV, you’re best off to take that rapid test when you first feel symptoms," Middleton added in a university news release.

She and her colleagues took what they knew about Omicron virus variants of the COVID-19 virus, typical patient behavior and other factors, and they created a computer model to gauge when testing might be most accurate.

They found that if you take a COVID rapid test very soon after symptoms begin, the test misses infection 92% of the time. Taking the test two days after symptoms arise knocks that false-negative rate down to 70%, and your chances of identifying COVID accurately rise even higher on day three, the team said.

On day three after symptoms begin, the test will catch about a third of infections.

That may still seem like a high false negative rate, but the authors noted that COVID tests are really designed to spot those folks with "high viral loads."

“Diagnosing only one third of infections can still cut transmission substantially if we've diagnosed the most infectious third," explained study senior author Daniel Larremore. He's a professor of computer science at the UCB's BioFrontiers Institute.

Why the lag with detecting COVID?

As the researchers explained, most everyone by now has been exposed to the COVID virus so their immune systems do react quickly, and this will trigger symptoms.

However, unlike other common viruses, newer variants of COVID grows relatively slowly, taking longer to be accurately picked up on tests.

In contrast, viruses like the flu or respiratory syncytial virus (RSV) replicate quickly, so tests should pick them up soon after symptoms start.

“If you’re trying to make a decision about whether to go to book club or go to Bingo night with the grandparents, testing is a really good idea,” said Larremore. “But COVID has changed.”

Larremore also believes that differences between the viral growth rates of flu, RSV and COVID make using new "three-in-one" virus home test kits problematic.

“This is the conundrum,” said Larremore. “If you go in right away and test for all three, you can learn a lot from the flu and RSV tests, but you may have swung too early for COVID. If you wait a few days, the timing might be right to catch COVID, but you are too late for flu and RSV.”

Middleton believes that recently updated U.S. Center for Disease Control and Prevention testing and prevention guidelines -- where folks test themselves again before determining whether its safe to return to work and socialize -- make more sense these days.

“The five-day isolation policy made people isolate for too long in most cases,” Middleton said.

The study was published recently in the journal Science Advances.

More information

To find out more about COVID-19, head to the CDC.

SOURCE: University of Colorado Boulder, news release, June 24. 2024