Rapid Covid tests are being used more widely: Can they be trusted?

Britain and France agreed this week to use on-the-spot rapid Covid-19 tests to end a 48-hour block on movement between the two countries, the latest adoption of a technology that could become a crucial tool to controlling the spread of coronavirus in 2021.

The UK government has spent at least £1.5bn on so-called lateral flow tests and has already provided the simple devices to local authorities, care homes, hospitals and universities. The UK regulator went further on Tuesday, granting the tests an emergency authorisation for use by members of the public with no medical training.

Proponents of lateral flow tests say the devices, which can deliver a result in 20 minutes, will significantly boost the UK’s testing capacity and are a useful tool for finding people who are currently infectious. From next month, the tests will be deployed across Britain’s schools to identify cases of the virus before the start of the new term.

“Testing asymptomatic people is an enormous priority,” said John Bell, Regius professor of medicine at Oxford university, who has overseen studies into the accuracy of these tests for the government. “If there’s no testing, they don’t get caught. Every successful positive you get is a win.”

But several scientists have remained sceptical despite the regulator’s approval and increasingly widespread uptake, arguing that the public has been misled about the tests’ accuracy.

Unlike RT-PCR tests, which look for Covid-19’s genetic material, lateral flow tests identify a protein antigen on the virus’s surface. Lateral flow devices do not amplify the virus (unlike RT-PCR tests) and therefore struggle at detecting lower levels of infection.

Several analyses have found that the lateral flow test developed by Innova — the biggest supplier to the UK government and one of a handful of tests that has met the government’s criteria for accuracy — picked up less than half of active infections.

“Schools are going to be a nightmare,” said Tom Lewis, clinical lead for pathology at North Devon District Hospital. “If you’re relying on this to pick up cases, you’re going to miss more than half of them.”

Professor Tim Peto at the University of Oxford disagreed. “They’re not perfect, but that doesn’t stop them being a game changer for helping detect cases of infectious disease,” said Prof Peto, who has worked with Prof Bell on validating the tests’ accuracy.

Their analysis found that the Innova tests pick up between 84 and 96 per cent of infections in people with a high viral load, making it ideal for identifying people who are currently infectious. The study also concluded that using the devices to identify and then quarantine cases of Covid-19 can reduce transmission by between 67 and 90 per cent.

This means the test is most effective in catching people who are infectious, and less effective at identifying people who have a low viral load, who are normally at the start or the end of their Covid-19 infection. In contrast, the “gold standard” PCR tests detect all forms of the virus.

The best source of data for widespread use of rapid tests to plot infection and reduce transmission comes from Slovakia, where two-thirds of the population — 3.6m people — were tested between October 31 and November 1 and over 57,000 cases were identified. 

Using lateral flow tests to identify and quarantine cases is estimated to reduce transmission of Covid-19 by between 67 and 90% © Matthew Horwood/Getty

An analysis of the campaign by researchers at the London School of Hygiene and Tropical Medicine estimated that the mass testing regime, coupled with quarantine measures, led to a 60 to 70 per cent reduction in the rate of infection across Slovakia.

“We see strong evidence that the combination of short lockdown and mass testing has led to a very rapid reduction in the burden of Covid-19,” said Stefan Flasche, professor of vaccine epidemiology at LSHTM and co-author on the paper.

The biggest pilot programme in the UK was conducted in Liverpool, where 25 per cent of the city’s 497,000 residents took a rapid test in November, leading to the identification of 897 positive cases, according to comprehensive data released on Wednesday. The Innova tests used were found to have picked up 40 per cent of PCR-positive cases, with two-thirds of those detected having a high viral load.

The academics who ran the study said that targeted rapid testing was both popular and valuable. Though they could not prove that the tests had led to a fall in infection rate, they noted that cases in Liverpool had fallen more quickly than in neighbouring Manchester.

The evidence suggesting that lateral flow tests are able to detect the virus when carriers are infectious suggests they may be particularly useful for making short-term behavioural decisions, such as whether to enter a care home to visit a relative. However, scientists largely agree that some pre-symptomatic cases will be missed, making it an imperfect tool.

“There is no perfect test, that’s the trouble, everything is a compromise” said Jo Martin, professor of pathology at Queen Mary University. “You have to know its limitations when you take it.”

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