When LFTs were introduced, they were criticised for being less accurate than PCR tests, which are analysed in a lab. But PCR tests are costly in comparison and therefore not seen as an effective way to carry out repeated testing.
Based on the UCL research, Prof Irene Petersen, lead study author, said people who get a positive LFT result “should trust them and stay at home”. But government guidance says people must get a follow-up PCR test after a positive LFT to confirm they have Covid - and they can end their self-isolation when they get a negative result in a PCR test.
The UK’s Health Security Agency is investigating the cause of some people receiving positive LFTs followed by a negative PCR, as has been widely reported recently, but said there was no evidence of any technical issues with test kits.
When the researchers used a new formula for calculating the test’s accuracy, they found LFTs were more than 80% effective at detecting any level of Covid-19 infection and likely to be more than 90% effective at detecting who is most infectious when they use the test.
This is much higher than previously thought, they say, adding that rapid tests are “a very useful public health tool” for stopping the spread of the virus.
Prof Michael Mina, from Harvard School of Public Health, also part of the research team, said the LFTs could “catch nearly everyone who is currently a serious risk to public health” when viral loads are at their peak. “It is most likely that if someone’s LFT is negative but their PCR is positive, then this is because they are not at peak transmissible stage,” he said.
There was much criticism of the rapid tests when they were first trialled in Liverpool last year because they were directly compared to PCR tests, which were often described as the gold standard.
Lateral flow tests and PCR (polymerase chain reaction) tests do different things:
- LFTs pick up the most infectious people by detecting material from the surface proteins of the virus
- PCRs detect genetic material of the virus which can be present in the body for several weeks after somebody is actually capable of passing it on
The UCL peer-reviewed study concludes that criticism of LFTs for low sensitivity “have reached the wrong conclusions”, “confused policy-making” and “damaged public trust in LFTs”.
Dr Sophia Makki, incident director for Covid-19 at the UK Health Security Agency, said: “Around one in three people who have Covid-19 never show any symptoms. Using LFDs (lateral flow devices) help to find asymptomatic cases who have a high viral load and are most likely to pass on the virus to others.”