The UK Health Security Agency's chief medical advisor said it was aware of 'some areas' reporting discrepancies between lateral flow and PCR results and was looking into the issue.
The agency said there was no evidence that a new variant of COVID-19 was responsible - and that there was also no evidence of 'technical issues' with PCR testing kits.
PHE said its general advice is that people should avoid mixing with others if they feel unwell - and advice on the NHS website says: 'If you still feel unwell after a negative test, stay at home until you're feeling better.'
PCR COVID-19 test
However, some GPs called for guidance to be updated amid concerns that potential false negative PCR results could lead people infected with COVID-19 to believe they did not need to isolate.
UK Health Security Agency chief medical adviser Susan Hopkins said: 'We have been made aware of some areas reporting positive LFD test results with subsequent negative PCR tests and we are looking into the cause.
'If you get a positive LFD test, it’s important to make sure that you then get a follow up PCR test to confirm you have COVID-19. If you have symptoms of COVID-19, self-isolate and take a PCR test. With any test it is important to carefully read and follow the instructions for use on the test kit so as to avoid any incorrect readings.'
Although lateral flow devices are understood to be less sensitive - and therefore less likely to pick up COVID-19 cases than PCR tests - false positives are extremely unlikely. The DHSC reported earlier this year that 'for every 1,000 lateral flow tests carried out, there is less than one false positive result' - explaining why the reports of negative PCRs after positive lateral flows have caused concern.
Bristol GP Dr Shaba Nabi wrote on Twitter that her son had classic COVID-19 symptoms and had 'lit up' lateral flow tests 'like the Blackpool lights' but returned a negative PCR result - and that 'lots of school kids' in her area had the same experience. 'What on earth is going on,' she asked.
My son has a cough, fever and anosmia.— Shaba Nabi (@ShabaNabi) October 8, 2021
His LFT lights up light the Blackpool lights. Yet his PCR is negative.
This is the experience of lots of school kids in my area
What on earth is going on?
A hospital consultant in the same area said here had been 'many reports' of this issue in the area, and that some people had been advised to 'carry on as if they didn't have COVID-19' after negative PCR tests.
Reports have suggested the issue has been more common in south-west England, but GPs in Manchester and Oxfordshire also reported seeing similar discrepancies between lateral flow and PCR results.