A third of secondary school students who tested positive using rapid Covid tests at home received negative results when checked with a follow up laboratory based PCR tests, new figures show.
The Department for Health and Social Care has produced new data showing the proportion of secondary students' home lateral flow tests which give positive results are confirmed by lab-based PCR tests.
The results show that since schools returned more than 1,000 pupil rapid tests appear to have returned false positives - with these students then testing negative when taking a PCR test, which are seen more reliable.
However the data shows that in recent weeks two-thirds of the secondary school students who tested positive for the virus at home also tested positive when a follow up PCR test was done to check their results.
And in one week more than 80 per cent of lateral flow test positives were backed up by PCR tests detecting Covid-19.
Government data shows that from April 8 to 14 there were 864 positive test results from secondary school students taking a lateral flow test at home.
Of these 510 were not matched to a PCR test. However of the 354 rapid tests that were matched to a PCR test 237 tested positive again while 111 tested negative.
This means that 67 per cent of the positive results from rapid home testing were backed up by PCR tests.
From April 1 to April 7 there were 1,094 positive tests from secondary school students using lateral flow tests at home. Of these 591 have been matched up to a PCR test of which 486 were found to be positive while 98 were negative.
This means 82 per cent of the positive results from rapid home testing of secondary school students were backed up by PCR tests.
In the preceding week, 929 out of 1,274 positive rapid tests matched to a PCR test were confirmed to be positive and 330 were found to be negative.
How often rapid home Covid test positives are backed up by PCR tests
Jon Deeks, Birmingham University's professor of biostatistics said: "These results do show the importance of students' home testing being checked with a PCR tests because the results show there is a significant fraction of tests that are producing false positives.
"What these figures don't tell us is the proportion of false negatives that are produced by lateral flow tests and that remains a concern.”
When schools reopened to all pupils in March there was confusion about the role of PCR testing for students who had already tested positive using a lateral flow device.
Children's minister Vicky Ford said that children who test positive for Covid with a lateral flow test but subsequently receive a negative PCR result should not return to class.
However, 10 Downing Street then corrected this and said that students who get positive rapid test results at home will receive a follow-up PCR test. And if they test negative, then they will be able to return to school.