Coronavirus labs have been told to test patients twice in some circumstances amid concerns that false-positive tests could be exaggerating the national picture.
Public Health England has released new guidance to laboratories advising them to re-run samples if the positive result is uncertain.
Experts have warned the new guidance may have been issued due to varying standards used across testing sites, particularly at the Government’s flagship Lighthouse labs.
Previous reports have raised concerns of staff working in these private labs who say that standards hugely differ between sites.
Patients will be advised to self-isolate pending the results of a second test.
But contact tracing should not be started until a second strong positive result is returned from the repeated sample, the guidance says.
“It may be because there's been these mass tests that have been done through the Lighthouse labs... maybe they're not adhering to (repeat testing for low positives) because they are not part of the routine NHS ways of doing things,” said Professor Deenan Pillay, a virologist at University College London and a member of the Independent Sage committee.
Prof Pillay added that these labs are not set up in the same way as NHS ones and could lack “oversight” for these sorts errors, which are standard in viral testing.
“I can say for the lab that I am associated with at UCH that when there is very, very low levels of virus identified that's not called a positive, but whether that's the case at Lighthouse labs I’m not sure,” he said.
The impact of false positive results on national levels will be more significant as the prevalence virus goes down within the community.
If a few false-positives are reported when the prevalence of the virus is high, it is unlikely to have much of an impact.
But if the prevalence of cases is low then a false positive will become “much more noticeable”, said Prof Pillay, and could skew the national picture.
It comes as the weekly number of positive Covid-19 cases in England in late August was the highest since the end of May, according to the latest data from NHS Test and Trace.
PHE is now advising labs to set a threshold for what could be determined as a false-positive when the presence of the virus is very low.
It suggests that until now there has not been a standard baseline across all testing sites to determine what would equal a strong positive result.
Prof Pillay said the new guidance will impact testing turnaround times as technicians may now be repeating more samples than before, but he added “that is something that is not an unusual thing to happen for other infections”.