Lateral flow tests (LFTs) are very good at detecting people most likely to spread Covid-19 and positive results should be trusted, say University College London researchers.
When LFTs were first introduced, they were chastised for being less accurate than PCR tests, which are run in a lab.
However, the study discovered that rapid tests were “a very useful public health tool” for halting the virus’s spread.
One third of people with Covid can spread it while showing no symptoms.
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.
There have been recent reports of this happening in south-west England, leaving people unsure whether to isolate or not.
The UK’s Health Security Agency said it was looking into the cause, but there was no evidence of any technical issues with test kits.
Prof Petersen said: “When [Covid is] more common, there is no need to confirm it with a PCR – it’s more likely it is a positive,” she said.
When the researchers used a new formula to calculate the accuracy of the rapid test, they discovered that LFTs were more than 80% effective at detecting any level of Covid-19 infection and are likely to be more than 90% effective at detecting who is most infectious when they use the test.
They claim that this figure is much higher than previously thought.
Prof Michael Mina of Harvard School of Public Health, who was also a member of the research team, stated that when viral loads are at their peak, the LFTs could “catch nearly everyone who is currently a serious risk to public health.”
“If someone’s LFT is negative but their PCR is positive, it is most likely because they are not at peak transmissible stage,” he says.
Rapid tests are widely used in schools, workplaces, and for allowing entry to large events to test those who do not exhibit any symptoms.
‘Apples and oranges’
Since they were introduced in secondary schools in England in March, 103,409 LFT tests have come back positive, with 79,000 matched with a confirmatory PCR and 69,500 of those confirmed positive, according to NHS Test and Trace data (and 7,647 came back negative).
When the rapid tests were first trialled in Liverpool last year, they were heavily criticised because they were directly compared to PCR tests, which were widely regarded as the gold standard.
“This is like comparing apples and oranges,” Prof Petersen said.
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
According to the peer-reviewed UCL study, criticism of LFTs for low sensitivity “has reached the wrong conclusions,” “confused policy-making,” and “damaged public trust in LFTs.”
According to the researchers, who published their findings in Clinical Epidemiology, health professionals and the general public should be aware of what the tests do.
And they acknowledge that errors in how people take the tests or how they are processed in the lab could affect the results – factors that were not considered in their study.
The current government guidance says that if you receive a negative follow-up PCR test result, and this PCR test was taken within two days of the positive LFT, you will be told by NHS Test and Trace that you can stop self-isolating.
However, it states that if the PCR result is positive, you choose not to take a follow-up PCR, or the test was taken more than two days after the positive LFT, you must continue to self-isolate.
“Around one in three people who have Covid-19 never show any symptoms,” said Dr Sophia Makki, incident director for Covid-19 at the UK Health Security Agency.
The use of LFDs (lateral flow devices) aids in the detection of asymptomatic cases with a high viral load who are most likely to spread the virus to others.”