Experts have warned that people should swab both their throat and nose when conducting Lateral Flow Tests (LFTs), as anecdotal evidence has demonstrated that nasal samples alone may be less effective at identifying positive cases of Omicron.
Dozens of people have reported testing negative on LFTs when only swabbing their nasal passages, including after receiving a positive PCR test result.
Prof Irene Petersen, an epidemiologist at University College London, said she had been “trying to get an understanding of what’s going on” after seeing the emerging evidence.
She believes that nasal swabs may be less effective at picking up positive results for those in the early stages of Omicron infection because it “may replicate in your throat and then your nose”.
Well, there it is. Today, with the “wrong” (i.e. cold) symptoms and after a string of negative LFTs, I finally took Twitter advice and swabbed my throat as well as my nose (no mean feat with that diddly stick). If you think you might have COVID, consider adding the throat sample pic.twitter.com/YKihOKh6mE
“To me it seems as though some people may have an infection in their throat before they get an infection in their nose,” she said.
“It seems a question about how Omicron establishes itself in your body and that appears to be different than previous variants.”
She added the fact that many people reported sore throats among early symptoms of the variant could be related.
“You want to catch the virus as early as possible, but that may be from your throat rather than your nose,” said Prof Petersen.
LFT tests: Nose only doesn't work! Yesterday throat + nose (my only symptom was a sore throat). Today (after POSITIVE PCR) tried nose only (per leaflet). NEGATIVE! Same test done same time with nose + throat POSITIVE. How many people out there think they're clear? #covidchristmas pic.twitter.com/KNyI6nZpFE
As a result, she suggested it was “a good idea” that people ensured they took samples from both even if not directed to on instructions.
“Some tests are specifically for both throat and nose, some are just for your nose. But I can’t see why you couldn’t do both your throat and nose,” she said.
Professor Tim Spector, lead scientist on the Zoe Covid study, also echoed the sentiment. “As well as knowing about cold like symptoms – always swab both your throat as well as your nose if you want an accurate LFT,” he said earlier this week.
While the discrepancy in accuracy between nasal and nasal and throat swabs could lead to many people believing they are not infected when they have the virus, Prof Petersen said it would cause less of an issue in terms of those testing to get out of isolation.
“When you are really infectious you have millions of copies of the virus in different tissue in your body, and nasal swabs should be able to pick this up,” she said.
“It may be less of a problem at the other side of infection than at the beginning. Most people would not be infectious after seven days.”
Another Important public health message! As well as knowing about cold like symptoms- always swab both your throat as well as your nose if you want an accurate #LFT ! We are lucky in U.K. to have them – so let’s use wisely https://t.co/wsjPgC4VPj
She stressed that people should continue to test regularly using LFTs, including taking one just before socialising with other households as Omicron “is incredibly fast”.
In practice, this means that individuals with the variant could not have enough viral load to test positive in the morning, but be fully infectious by the evening.
However, it comes against a backdrop of ongoing shortages of lateral flow tests.
On Wednesday, an industry body revealed that pharmacies in England did not receive any deliveries of LFTs for four days over Christmas.
Meanwhile the home testing kits were also unavailable on the government website for a third day running.
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