Will false-positive Covid tests slash school attendance?

School children across England are being tested for Covid after returning to England’s schools this week. But while the government is hailing the new mass testing regime as a key part of Boris Johnson’s roadmap for easing lockdown, scientists have spotted a flaw in the plan.

All secondary school children are being offered three lateral flow tests on their return, before being asked to carry them out twice a week at home.

However, while the tests provide a result “on the spot”, these results are “less accurate” than those from the PCR tests that are used at official testing centres and then sent off to labs, says BBC health correspondent Nick Triggle.

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And “assuming most pupils take up the option” of the lateral flow tests, “that could result in thousands of pupils and their close contacts being asked to isolate unnecessarily”, Triggle continues.

Research shows that false-positive tests are more common than false-negatives and occur “one to three times for every 1,000 tests carried out”. So in a worst-case scenario, for every million students tested, 3,000 would be sent home despite not having Covid.

Concerns about inaccurate results have been heightened by confusion over whether children found to have received a false positive will be allowed to go back to school.

The Telegraph reports that the mass testing policy risks “descending into chaos” after Children’s Minister Vicky Ford suggested yesterday that students who test positive from a lateral flow test would have to self-isolate even if they got a subsequent negative result from a PCR test.

Johnson’s spokesperson later said that pupils who get positive lateral flow tests at home but tested negative in a follow-up PCR test could return to class.

However, government officials then clarified that pupils “who get a positive lateral flow test at school will be banned from lessons for ten days even if they get a subsequent negative PCR test”, the paper says.

Experts have criticised the rules as illogical, and point out that as coronavirus cases in the community fall, the number of true positives will go down but the number of false positives from lateral flow tests will remain constant - skewing data on infection rates.

Professor Jon Deeks, an expert in biostatistics at Birmingham University, told that BBC that “any testing system is a trade-off between benefits and harms”.

But “my concern is that it will actually cause more harm than good”, Deeks said.