Seven in 10 positive coronavirus tests in school children are likely to be wrong, experts have warned, as they called on the Government to bring back confirmatory PCR testing.

Biostaticians are concerned that infections in the community are now so low, that false positives …. are vastly outnumbering true cases, leading to real cases being missed and families needlessly being asked to isolate.

Pupils are currently being tested twice a week for coronavirus using lateral flow devices, but real-world data has shown they miss positive cases around 50 per cent of the time.

Similarly, although they pick up 99.9 per cent of negative cases, meaning fewer than 0.1 per cent will be false positives, the prevalence rate is now low enough that false positives will be making a significant contribution to the overall figure.

Previously, positive tests were confirmed using a more accurate polymerase chain reaction (PCR) lab test, but this week the Government admitted it had quietly scrapped the process at the end of January.

It means that thousands of pupils and their families are having to isolate needlessly, and missing more school after months away from classrooms.

Latest figures from NHS Test & Trace show that in secondary schools there were 328 positive cases found in 663,332 tests between February 25 and March 5.

It means that just one in 2,000 pupils are testing positive for the virus. Experts warn that is far lower than the 1 in 270 that the Office for National Statistics (ONS) believes to be the current community prevalence, suggesting large numbers of cases are being missed.

In primary schools 613 positives found from 721,546 tests in primary schools giving an overall school prevalence rate of 0.067 per cent or roughly 1 in 1,500.

With that prevalence rate, if a million tests were conducted 670 should come back positive.

With current testing accuracy, half positive cases (335) would be missed and 1,000 would be false positives. It would mean that out of a total of 1,335 positives, just 25 per cent would be accurate.

Professor Jon Deeks, a biostatistician from the University of Birmingham, said: “We would expect far more false positives than true positives amongst those testing positives in schools.

“There are many uncertainties but given the DHSC data it seems likely that over 70 per cent of positive test results are false positives, potentially many more.  Addition of a confirmatory PCR would add little cost and would most likely reduce false positives to 1 in 1,000,000.

“The refusal to confirm lateral flow results with PCR is at best perplexing, will make testing less attractive, and create harm by wrongly isolating individuals, families and other close contacts.”

Last night, the Royal Statistical Society (RSS) wrote to the Medical and Healthcare Products Regulatory Agency (MHRA) warning of the ‘potential harm’ of failing to re-test children.

The RSS warned that there could be ‘legal implications’ for curtailing the freedom of children, classmates and family members on such a flimsy scientific basis and asked for the MHRA to insist on a confirmatory PCR test.

Sage warned false positives would become an issue

In a report to the Scientific Advisory Group for Emergencies (Sage) last year, Dr Carl Mayer and Dr Kate Baker, from the Government Office for Science, warned that Britain would face a problem with false positives once the infection rate became very low.

“When only a small proportion of people being tested have the virus, the operational false positive rate becomes very important,” they wrote in their report.

They warned that there was risk of overestimating the Covid-19 incidence, and the demand on test and trace.

Profesor Carl Henegehan, director of the Centre for Evidence Based Medicine at Oxford University said: “This becomes a huge problem when you have low prevalence. We have been working for years to try and minimise this problem of false positives in mass testing programmes.

“Once you’re doing the levels of testing we’re doing, you’re false positive threshold will exceed the test positivity threshold and so testing becomes of limited value.”

This week, the Department of Health said the accuracy of lateral flow tests was 99.9 per cent and could be as high as 99.97 per cent. But even at the upper end, the chance of a test being correct with the current school prevalence rate would only be around 50 per cent.

Prof Sheila Bird, formerly programme leader of the MRC Biostatistics Unit at University of Cambridge, said: “NHS Test & Trace and Public Health England have attempted to justify not offering PCR during school-monitored use of lateral flow devices for asymptomatic screening of secondary pupils on their return to school in England.

“More than 600 positives can be expected this week when over 1.5 million asymptomatic secondary pupils have been screened.

“Half may be PCR-negative. By withholding data and denying PCR-confirmations, the half who are PCR-negative will never know – unless families and schools demand fairness for our children.”