Rapid lateral flow tests 'should not be used for test and release'
Rapid lateral flow tests for Covid do not work well in people with no symptoms and should not on their own be used to allow people to go to work or school or to travel, experts have said.
The UK has bought millions of rapid tests which give results within half an hour. Teachers, schoolchildren and their families without any symptoms are being asked to test themselves using the kits twice a week.
“About one in three people with coronavirus do not have symptoms but can still pass it on to others,” says government guidance. “Regular testing of people without symptoms is important to help stop the virus spreading and protect your loved ones.”
However, a Cochrane review, carried out by a team of international, independent experts, has found that rapid antigen tests – known as lateral flow – correctly identify on average 72% of people who are infected with the virus and have symptoms and 78% within the first week of becoming ill. But in people with no symptoms, that drops to 58%.
But there were big differences between brands. In people with symptoms, SD Biosensor, which has been approved by the World Health Organization, picked up 88% of infections, while the Innova test, used very widely in the UK since mass testing in Liverpool, picked up 58%.
Little data exists to show how well the tests perform when the person has no symptoms. For Innova, said Jon
Deeks, professor of biostatistics at the University of Birmingham and one of the authors, there are only two studies.
The Liverpool pilot project showed the test found 40% of people with Covid, and in a very small study he conducted in students at the university, it picked up just 3%.
About 40m of the tests have been given out, he said, but in those two studies a total of just 78 people were identified as having 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 said.
There have been no studies on the use of the tests in schoolchildren with no symptoms, he said. “We have no data on the accuracy of these tests in children.”
The team identify three possible uses of the tests: testing to detect infection; testing to release people from selfisolation or quarantine; and testing to enable them to go to school, work or an event.
“You can’t do such a test for testto-release or test-to-enable,” said Deeks. People would go back into the community and be socially mixing. The tests, he said, “will probably detect about half the cases”.
They create both false negative results – when people are wrongly told they are not infected, and also false positives, when people will unnecessarily be instructed to self-isolate, which could mean they cannot work.
In a mass testing situation, said Deeks, among 10,000 people with a prevalence rate of 0.5%, similar to now, you would expect 50 people to have Covid. The tests would pick up 35 of the 50, but 90 would wrongly be told they were infected.
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.”
The tests do have a use among people with symptoms, said the scientists. At a test centre, a hospital or GP surgery, they can give a rapid diagnosis, which means contacts can be traced faster than at present. The positive test must then be confirmed with a PCR swab test, however, they say.
The government, however, maintains that the tests are an essential part of the Covid response.
Dr Susan Hopkins, Covid-19 strategic response director at Public Health England and chief medical adviser to the Test and Trace programme, said: “As this report highlights, rapid tests are effective at detecting Covid-19 in people that are highly infectious, both with and without symptoms.
“They are an absolutely crucial way to help bring down infection rates and keep them low.
“Every day, rapid testing is helping us find cases of Covid-19 that we wouldn’t otherwise know about, breaking chains of transmission and potentially saving lives. Please do take a test if you’re offered one – the more we test, the more cases we will find.”