BERKELEY -- False-negative rapid COVID tests are on the rise.
When Carl Berger got sick earlier this year, he figured he had COVID-19 because his wife had the virus.
"I think I had a headache and I generally felt kind of weak. I had contact with my wife, who is known to be positive - So I figured there was a good likelihood that I was positive," said Berger.
He took a rapid test at home. It came back negative.
"I was skeptical," said the East Bay man.
A few days after that, both a rapid test and PCR came up positive for COVID-19.
A study published by the Harvard School of Public Health in April noted that some strains were harder to locate using a rapid test than others.
"The delta variant was more difficult to detect, it had a higher limit of detection than omicron. This may relate to the omicron subvariants because some of the strains of the omicron subvariant do have mutations in the gene that's detected by the antigen test," said Ph.D. candidate Sydney Stanley.
Dr. James Kirby of the Harvard School of Public Health says just like vaccines are getting new strain updates, so should tests.
"If we look at the original antigen tests that are out there, they just have data for the original Washington strain," said Kirby
It's a sentiment echoed by Dr. John Swartzberg of UC Berkeley.
"Another thing we learned from that study is that we have to constantly be re-looking at the rapid tests to make sure as this virus spins off new variants and subvariants is that the rapid tests work as well against the new ones as they did against the old ones," said Swartzberg.
He says don't be surprised if your first rapid test comes up negative when you have symptoms.
"Often, early in the course of illness, the rapid test is negative when you actually have that disease," Swartzberg said.
There are other theories on why false negatives are on the rise, including that the viral load hasn't reached contagious levels and that vaccines are doing what they are supposed to do: keeping viral loads low.
Health experts advise people who have symptoms to isolate until they get at least three negative rapid tests over the course of several days.
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