Senior government officials have raised “urgent” concerns about the mass expansion of rapid coronavirus testing, estimating that as few as 2% to 10% of positive results may be accurate in places with low Covid rates, such as London.
Boris Johnson last week urged everyone in England to take
two rapid-turnaround tests a week in the biggest expansion of the multibillion-pound testing programme to date. However, leaked emails seen by the Guardian show that senior officials are now considering scaling back the widespread testing of people without symptoms, due to a growing number of false positives.
In one email, Ben Dyson, an executive director of strategy at the health department and one of health secretary Matt Hancock’s advisers, stressed the “fairly urgent need for decisions” on “the point at which we stop offering asymptomatic testing”.
On 9 April, the day everyone in
England was able to order twice-weekly lateral flow device (LFD) tests, Dyson wrote: “As of today, someone who gets a positive LFD result in (say) London has at best a 25% chance of it being a true positive, but if it is a self-reported test potentially as low as 10% (on an optimistic assumption about specificity) or as low as 2% (on a more pessimistic assumption).”
He added that the department’s executive committee, which includes Hancock and the
NHS test and trace chief, Dido Harding, would soon need to decide whether requiring people to self-isolate before a confirmatory PCR test “ceases to be reasonable” in low infection areas where there is a high likelihood of a positive result being wrong.
The accuracy of rapid coronavirus tests and how they should be deployed have been the focus of months of
debate in the UK. The proportion of false positives – people incorrectly told they have the virus – increases when the prevalence of the disease falls. This happens because although the number of true positives is falling, the tests produce roughly the same number of false positives – meaning the proportion of incorrect results becomes greater.
It means thousands of people could be wrongly told to self-isolate and miss out on earnings or education due to inaccurate results. The government has advised anyone who tests positive with a rapid test to take a follow-up PCR test and self-isolate until they receive a negative result – but
some experts have said this process is too slow and that a second lateral flow test would be as likely to produce the correct result.
Figures produced by government officials estimate that currently only one in 10 positive results are likely to be accurate in London and south-east and south-west England, where there is less Covid-19 in circulation. In England as a whole, they estimate that only 38% of self-reported tests are thought to be accurate, based on the current prevalence of the disease. The Guardian has also learned that Public
Health England (PHE) raised concerns about the plan for mass testing, days before it was announced on 5 April.
https://www.theguardian.com/world/2...gland-may-be-scaled-back-over-false-positives
