As testing rules are set to shift in England, with asymptomatic people no longer having to take a follow-up PCR after a positive LFT, scientists have also reemphasised the rarity of false positives.
“False negatives (i.e. testing negative despite actually having Covid-19) are common, whilst false positives are in fact very rare,” said Dr Michael Head, a senior research fellow in global health at the University of Southampton.
“Therefore, if a lateral flow test comes up with a positive result, then it’s important to assume a case of Covid-19 and to follow the guidance accordingly. This applies even if a second lateral flow is negative.”
A recent Government analysis found that for every 1,000 lateral flow tests carried out in the community, fewer than one brings up a false positive.
There have been several reports of people testing at home receiving both a positive and a negative result within a matter of hours.
Alison Clarke, 58, told i how she recently tested positive in the morning and negative in the evening on three consecutive days with LFTs, while waiting to leave her isolation period between days seven and 10 after receiving a positive PCR result.
However, Professor Azeem Majeed, head of primary care and public health at Imperial College London, said that as lateral flow tests are “less sensitive” than PCRs, it was “not surprising” that people could get different results on the same day.
He urged anyone in an isolation period who has done an LFT to treat any positive result as signalling they still infectious for that given day.
Professor Majeed said said the most likely explanation for why the tests were throwing up different results was that they were mot being carried out correctly.
“I don’t think there is an association between the time of day the test was carried out and the chances of it being positive,” he said.
“The key thing is the correct technique – it has a big affect on the result. Make sure you don’t eat for a little while beforehand, make sure you blow your nose and wash your hands.”
The reason behind these rules is to ensure a “clean sample”, he added. “If you’ve still got food [residue] in your mouth, you could just coat the swab with that and that could affect the result. You could also pick up a bit of snot, and that might obscure the result.”
He added: “I also advise swabbing the throat as well as the nose [despite LFT kit instructions to only swab your nose], on the grounds that if you swab from more sites you’re more likely to pick up infection than by swabbing the nose only.”
To increase the chances of a more accurate result, swab for at least 10 seconds on both tonsils, and then in each nostril, while “turning the swab around”, the professor suggests.
He emphasised that any second line seen on an LFT, even a faint one, is a marker of a positive test.
“Even if it’s very faint, that’s still a positive. Sometimes you get a bright red line, but even a faint line is positive – it means it’s picked up some of the viral proceeds,” said Professor Majeed.