As schools in England prepare to welcome all children back on Monday, staff and parents are getting ready for a new coronavirus testing regime.
So what do we know about the “lateral flow” devices they’ll be using? Should we be worried about false negatives or false positives? And will asymptomatic testing be enough to keep the epidemic at bay?
What’s the new scheme?
When they go back next week, secondary school pupils will take three tests at school “under the supervision of a trained operator”, according to the latest government plans. They’ll be expected to carry out a fourth test themselves using a home test kit.
After that, they’ll be asked to do a home test twice a week and report the results online or by phone to Test and Trace straight away.
“Primary school pupils will not be asked to test at this time”, the government says.
All school staff will be asked to take twice-weekly tests. And anyone who lives with a pupil or staff member will be able to get tested at the same frequency.
Could the tests give false reassurance?
The programme uses “lateral flow” tests, which give results in less than an hour – so they don’t have to be sent off to a lab. But as FactCheck reported in December, there’s some big question marks over the reliability of lateral flow tests compared to “PCR” lab tests.
The latest evidence suggests that lateral flow tests only pick up around 60 per cent of positive cases. That means that some people will be told they don’t have coronavirus, when in fact they do – so-called “false negatives”.
Dr Alexander Edwards, associate professor in biomedical technology at the University of Reading, told us this week: “We do know the rate of false negatives is very significant.”
But, he says, “the messages from public health officials continue to be clear (I hope) that even if you test negative with a COVID-19 test, it still doesn’t ‘prove’ that you are safe and not infected.”
He adds: “This is especially true with regular asymptomatic testing using lateral flow rapid tests. But it has always been true even for PCR [lab] testing of symptomatic patients that negative results are quite common and do not rule out COVID-19.”
Regarding lateral flow tests generally, Adam Finn, professor of paediatrics at the University of Bristol, explains: “These are ‘red light’ tests. If they come positive that means you are potentially infectious to others and must self isolate. They are not ‘green light’ tests. You cannot be sure that if the test is negative you are not infectious and you must continue to take the usual precautions.”
So even if their lateral flow tests show a negative result, kids, parents and staff will still be expected to socially distance and follow other restrictions.
Jose Vazquez-Boland, professor of infectious diseases at the University of Edinburgh, gave us his take. “Clearly, lateral flow antigen tests are less sensitive than PCR, their use involves a trade-off between speed and easiness of application and sensitivity of detection, and these limitations need to be understood and taken into consideration.”
Professor Vazquez-Boland told FactCheck that, in his view, “applying these less sensitive tests provides a valuable complementary tool to identify infected people in the population who otherwise would pass unnoticed or undetected if PCRs cannot be systematically implemented.”
He adds: “Although more prone to give false negative results because of their lower sensitivity, theoretically, [lateral flow] tests would in principle detect those individuals carrying/shedding a higher viral load and thus more likely to transmit the infection. If there is no other alternative, applying these tests is better than doing nothing.”
Is it safe to have secondary school kids administer their own tests?
As FactCheck reported in December, there’s evidence that the reliability of the lateral flow tests varies depending on who carries it out.
Researchers at Porton Down and the University of Oxford found that the tests were most accurate when carried out by lab scientists (picking up 79 per cent of positive cases), slightly less so when done by trained healthcare workers (73 per cent) and significantly worse when carried out by “self-trained members of the public” (58 per cent).
Though Dr Edwards points out that it’s still not clear whether the “self-trained members of the public” could improve over time. He says “It may simply be that people need some practice to operate these tests, but once they get the hang of the process they become more reliable. As home and community testing is still very new, we must work hard to make it as easy and effective as possible, and not assume it will ‘just work’.”
What about false positives?
There’s another risk that we have to think about with any type of test. How often does it wrongly tell someone without coronavirus that they are infected?
Professor Vazquez-Boland told us: “The risk with the less sensitive LFD tests is false negative results, not false positive ones. If the test is well designed and suitably specific, the rate of false positive tests should be very low.”
Dr Edwards agrees. He says “this is not so much of a concern” for two reasons. “The false positive rate seems to be quite low, so you might expect it would affect significantly less than 1 in 100 people.” Though he does note that when the true rate of infection in the wider population starts to fall, the number of people with false positive results will start to represent a bigger proportion of those testing positive.
The second reason he thinks false positives shouldn’t be too worrying is because we have ways to mitigate the risk. When someone tests positive on a lateral flow device, they’ll be asked to confirm the result with a PCR lab test. If that comes back negative, they won’t need to self-isolate.
Of course, that may mean some days away from school while they wait for the result, but Dr Edwards adds, “I hope individuals affected can understand it is far better for them to spend a few days studying at home, than for the whole class to become infected because of further spread. Or for the whole country to have to return to restrictions and home schooling.”
On balance, he says “it’s always going to be better to have most pupils in school with a few students isolating due to false positives, than having everyone home schooling because there is no other way to avoid spread.”
What effect could asymptomatic testing have on the overall epidemic?
Dr Edwards says: “Testing alone won’t help, but it is the combination of regular testing with other public health measures that has potential to slow spread and keep the number of cases as low as possible”.
He adds: “The intent for asymptomatic [lateral flow] testing in schools is to detect cases in the community as early as possible, and thereby direct rapid interventions (such as follow-up PCR testing and inform local public health teams) before spread.”
Professor Vazquez-Boland says “it will work alongside mass vaccination to reduce community transmission and keep coronavirus infection within manageable levels.”
Source – Channel 4