Why it might take longer to test positive on a COVID-19 test

In the past, a person might test positive for COVID-19 on a rapid test one or two days after developing symptoms. However, it may now take longer for positive test results to appear after symptom onset, with some people not testing positive until the fourth day after symptoms start.

According to Elizabeth Hudson, regional chief of infectious diseases at Kaiser Permanente Southern California, this delay in accurate test results is likely due to people having accumulated immunity to COVID-19, either from vaccinations or past infections.

"It's actually pushing back the time that people's COVID tests are coming up positive. So some people are testing at Day 1 and Day 2 and saying, 'Oh, it's negative, I don't have COVID,'" Hudson said. "If they probably tested themselves a couple of days later, there's a pretty good chance that it actually would turn out to be COVID."

In a post on X, formerly known as Twitter, Michael Mina, a public health expert and chief science officer for eMed, noted that existing immunity means that symptoms are likely to start quickly after someone is exposed to the coronavirus, even if their viral loads aren't that high yet. 

Another potential reason for a delayed positive result is that rapid tests were originally designed for earlier COVID-19 variants. Although they can still detect newer variants, a greater amount of virus samples may be needed to return a positive result.

Current recommendations for testing

If you are experiencing COVID-19 symptoms, CDC recommends that you test yourself immediately. If you receive a negative result and are still experiencing symptoms, the agency then recommends testing yourself against 48 hours later.

If your second rapid test is also negative, health experts taking another test four days after the onset of your symptoms. "They should really, probably on Day 4, retest themselves if they're doing the home antigen test," Hudson said.

If you're unsure of the results of your rapid test, you can also go to a medical provider to receive a lab-based PCR test. "That test is much more sensitive. And that [positive test result] would come up sooner," Hudson said.

Compared to at-home rapid tests, which can generally detect a coronavirus infection 80% of the time, PCR tests are more accurate and can typically detect the virus 95% of the time.

Overall, people who are feeling unwell are recommended to wear high-quality masks in crowded areas and to stay home to reduce the risk of viral spread. Health experts also recommend staying up to date with the latest vaccines, including for other respiratory viruses like influenza or respiratory syncytial virus.

"Given that hospitalizations are rising here because of holiday gatherings and decreased (waning) immunity, it is a good idea to take the usual precautions," said Nadia Roan, an immunologist at the University of California, San Francisco, and an investigator at the Gladstone Institutes. "We have a pretty big toolbox now compared to the beginning of the pandemic. They all work." (Lin, Los Angeles Times, 1/9; Vaziri, San Francisco Chronicle, 1/9)

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.