False results from lateral flow Covid-19 tests may even mean that more workplaces and schools are forced to close, even though the scheme was set up to keep them open, experts cautioned.
The mass testing programme has been rolled out across the education sector, some businesses and workplaces and health and care services.
But researchers said the tests could be put to better use in different scenarios, such as by paramedics treating unwell patients and at GP surgeries.
And if the rapid turnaround tests were also used at testing centres for symptomatic people, they could also reduce contact tracing by “around three days”, they added.
One academic said it was “shocking” that the Government based the mass testing on so little data.
As cases reduce in the population, a higher amount of false results will be expected and there will come a point when the testing strategy is “doing more harm than good”, experts cautioned.
The comments come after a new review of the data on rapid turnaround tests.
The Cochrane Review concluded rapid tests are better at finding infections among people with symptoms than people without symptoms.
The review, which contains data from 64 studies, concluded that on average, 72% of people with symptoms who had Covid-19 were correctly identified as being infected.
In people without symptoms, on average, the antigen tests correctly identified 58% of those who were infected.
They had a higher degree of accuracy for ruling out infection.
Speaking after the publication of the review, Jon Deeks, professor of biostatistics at the University of Birmingham, and an author of the review, said the tests could be used on a “test to detect” basis but should not be used for “test to release” or “test to enable”.
“These performance levels show that these tests can detect – so they can work in a test to detect role – and they will probably detect around half of the cases based on these estimates,” he said.
Prof Deeks said in a group of 10,000 people without symptoms, with a prevalence rate of 0.5% which is similar to recent studies on Covid-19 in the community, around 50 people would expect to have Covid.
But in theory, the tests would pick up 35 of the 50 cases.
And in the process, the tests could also create 90 false positives.
“In this example here, we actually are creating more false positives results than we are detecting real cases,” he said.
But Prof Deeks added: “These tests work well in symptomatic people and we should be looking at the role of these tests in that position, so this could be with paramedics attending people, in primary care and the testing centres where we could radically improve the speed of decision-making and contact tracing, which greatly needs to be done.
“A better role with these tests may be putting them in testing centres, where we would enable people to get those results before they leave a test centre – they work better for people with symptoms.
“If they were testing them in the testing centre, and our infection control teams were based in the testing centre, we can probably advance our contact tracing by three days, compared to what we’re currently doing.”
Prof Deeks said there are no studies which looked at the accuracy of using these tests in repeated testing strategies such as those in hospitals and schools.
He added: “The only data we have on how well that test works in asymptomatic (people) is based on the Liverpool and the Birmingham University studies – (in which) a total of 78 people had Covid.
“And so we’ve used that test – around 40 million tests given out – and all we have for the basis of how well it works is from 78 people who had Covid. So I personally find that quite shocking – the Government thinking that’s an adequate evidence base upon which to base such a large, expensive, and quite invasive policy for people to follow.”
He added: “When prevalence drops, you get more false positives and less true positives, that’s a mathematical certainty, and there’s a point where you should stop.
“We may already be below that point – it hasn’t really been established as to when is it that we’re actually going to be doing more harm than good.
“There is a point where the harm is going to outweigh the benefits of doing testing like this.”
Dr Ann Van den Bruel, associate professor of primary care at KU Leuven in Belgium and an author of the review, said: “The risk of the false positives in the screening setting is very high, and you may end up having the opposite effect of what you want to achieve and you may have to close more workplaces, more classes than what you’re currently doing without a clear effect on the epidemic, which is what we all want of course.”
Prof Deeks said that in schools last week, 2,500 tests were conducted to find just one case of Covid-19.
He added: “We don’t know how much the test costs, but if it was between £5 and £20, you’re spending up towards £100,000 to be able to detect one case. Now there are much better ways to spend £100,000 in our pandemic, to actually reduce the transmission more by improving for example the test and trace system.”