Rapid Covid Testing in England may be Scaled Back Over False Positives

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.

Full story at The Guardian