Positive Covid result from rapid test to be confirmed with PCR amid hunt for new variants

The move comes as part of efforts to quickly detect new and emerging ‘variants of concern’

Samuel Lovett
Tuesday 30 March 2021 18:10 BST
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The government is to use standard PCR testing to confirm positive Covid-19 results returned via rapid, on-the-spot tests which are now being used by millions of people every day across the UK.

This comes as part of efforts to quickly detect new and emerging “variants of concern”, some of which are partially capable of evading immunity triggered by infection or vaccination.

For those people who test positive via a lateral flow device (LFD), which is capable of returning a result in 30 minutes, the result will then be cross-checked using a PCR test.

These are more accurate than the LFDs and also make use of new technology, known as genotype assay testing, which could halve the time it takes to identify if a positive Covid test is caused by a variant of concern.

This will allow positive cases to be traced sooner and stop the spread of variants on UK soil, the government has said.

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Genotype assay testing is compatible only with PCR tests and not LFDs, meaning the latter is unable to detect or trace the spread of variants.

The UK has bought millions of LFD tests as part of plans to reopen society. Teachers, schoolchildren and their families without any symptoms are being asked to test themselves using the kits twice a week.

Contact tracing will continue to be implemented in the eventuality of a positive LFD result, but will be stopped automatically after receipt of a negative confirmatory PCR test.

NHS Test and Trace has introduced new features that will automatically inform anyone self-isolating from a positive LFD, along with their contacts, to stop isolating if the confirmatory PCR is taken within two days and is negative.

Jon Deeks, a professor of biostatistics at the University of Birmingham, said the new policy was “very welcome”.

“This will ensure that the risk that individuals are unnecessarily isolated through false positives is reduced,” he told The Independent.

“It is only a shame that it has taken so many weeks for concerns raised by the Royal Statistical Society and others to be addressed, with many children unnecessarily missing school as a result.”

A recent Cochrane review, carried out by a team of independent experts, found that lateral flow devices correctly identify, on average, 72 per cent of people who are infected with the virus and have symptoms, and 78 per cent within the first week of becoming ill. In people with no symptoms, this drops to 58 per cent.

At times of low prevalence, the probability of a false positive from an LFD is elevated, so the government is seeking to mitigate this by asking people to confirm a positive LFD result with a PCR test.

Prof Deeks, one of the authors of the study, said the LFD tests should not be used on their own to allow people to go to work and school or to travel.

Surge testing has also been used to help control the spread of new coronavirus variants. This involves ramping up testing and genomic sequencing in an area where a variant has been detected.

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