There are serious concerns about the Tory government’s 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.
Despite this fact, 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.
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).”
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.