The latest evidence about LFTs
Lateral Flow Tests (“LFTs”) usage may have waned in use in recent months, but they still have a place in health risk management strategies. However, the results they provide can vary greatly, depending on how they’re used.
However, the available evidence has kept on building, even as LFT usage has decreased. One independent body, Cochrane, exists to provide evidence in healthcare that can be trusted. Cochrane is a global independent network of researchers, professionals, patients, carers and people interested in health.
Cochrane regularly reviews available evidence on a huge range of topics. Its reports are always balanced, objective and highly respected amongst healthcare professionals. Cochrane has recently published a review of the available evidence surrounding Covid LFT outcomes.
The review comes after many well-structured trials continued to cast doubt on the accuracy of the tests. One large study by Imperial College London found that Innova brand LFTs missed Covid infections that were “substantial enough to be of clinical importance”. They were found to have missed up to 81% of Covid infections in one setting.
Cochrane originally published a summary of the available evidence in April 2021 and their latest report updates the considerable body of evidence that has been produced since. They aimed to show whether commercially available LFTs were accurate enough to diagnose Covid reliably.
The review included 155 studies evaluating 49 different LFTs produced by commercial test manufacturers. The results are based on 152 studies investigating a total of 100,462 nose or throat samples, including 16,822 with COVID-19 infection confirmed by a PCR test.
Cochrane found some weaknesses in the studies they reviewed. They could not always identify how long study participants had experienced Covid symptoms. LFTs were often interpreted without knowing the result of a PCR test (so a PCR result could not influence whether the test was deemed to be positive or negative). The tests were not always done in the way that the test manufacturers recommend. All this means that different results may be possible when LFTs are used in practice.
How accurate are LFTs in people with Covid symptoms?
Amongst people with PCR confirmed Covid infection, LFTs correctly identified Covid infection in an average of 73% of people with symptoms. This means they had a 73% test sensitivity.
The results were better when the tests were used in the first week after symptoms presented, correctly identifying 81% of infections. This compares to a 49% accuracy when they were used after the first week of symptoms.
LFTs correctly ruled out infection in 99.1% of people with symptoms who also had a negative PCR result.
How accurate are LFTs in people with no symptoms?
Amongst people who were confirmed to have Covid, LFTs correctly identified 55% of people who did not have any symptoms. The accuracy of the tests was higher when LFTs were only available to those who had been in close contact with a confirmed infection (an average of 64% were detected). The tests found 50% of confirmed cases when the tests were used amongst anyone who wanted to take one.
LFTs correctly ruled out infection in 99.5% of people without symptoms who also had a negative PCR test result.
How test usage affects accuracy
The accuracy of LFTs varied widely when results from different settings were compared (e.g. schools, healthcare settings, Covid test centres etc). The different results are likely to be related to the level of risk of exposure in different settings and the time since symptoms started.
Cochrane consider that there is not enough evidence to definitively prove how much the type of sample affects accuracy. It is not clear whether nasal sampling or nose and throat sampling provides the best results. Accuracy seems to vary with age, with sensitivity being lower in children.
The risks of getting an incorrect result
A negative LFT result in someone who has Covid could lead to increased transmission. This is particularly true if the negative result means they do not then adhere to social distancing and infection control steps. Positive results amongst those who do not have infection could mean unnecessary isolation and additional tests to confirm the result.
Which brands work best?
Very different results have been found amongst different tests, although it is difficult to prove which may be best, because very few studies directly comparing one brand with another have been done.
Only seven different test types met the World Health Organisation (WHO) threshold as “acceptable” for finding or ruling-out Covid infection, when people had symptoms. No tests met the WHO standard when they were used in people who did not have any Covid symptoms.
There is more information available in our “Which lateral flow tests work best?” feature from last year.
What do the results mean for businesses?
Amongst people who have Covid symptoms some LFTs are accurate enough to replace PCR testing. The are not as good at ruling out infection amongst people who have symptoms – false negative results are a very real possibility.
LFTs are far less accurate when used in asymptomatic people. 55% of the results were incorrect – not much better than tossing a coin. More evidence is needed to understand the impact of different testing strategies (e.g. in schools or at home).
Claims from test manufacturers that LFTs can identify the vast majority infections are likely to be overstated. The results may provide false reassurance, even though they are an important tool for mitigating risks.