Will rapid work-based Covid testing soon become the norm?

Pressure to relieve Covid-19 lockdown continues to ratchet up and in tandem with this a push to re-open workplaces by deploying a range of easily-used – and cheap – rapid lateral flow tests.

There is no doubt the technology, which mixes a nasal or throat swab with liquid on a paper strip and identifies antigens associated with Coronavirus infection, has improved. But reliability remains an issue.

In the UK, they are a central part of the government’s re-opening strategy; both before and after vaccination. Trials of lateral flow testing for teaching staff and older students have begun in schools before rollout after Easter.

Instant lateral flow tests are available free in London boroughs. Many employers, especially large multinationals and tech companies, are using them routinely in Britain – and throughout the world. The ideal scenario is: test yourself every morning, and head off to school or work, or even a concert, on the back of a negative result.

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A major UK study published last week, however, has exposed their shortcomings, notwithstanding prime minister Boris Johnson’s controversial suggestion employees should return to their workplaces.

What has the latest evaluation found?

The Cochrane Review – carried out by international experts – found lateral flow tests don’t work well in people with no symptoms and should not on their own be used to allow people to go to work or school or to travel. The bottom line is lateral flow tests should not be used for “test and release”.

Highly sensitive PCR tests are the most reliable way of detecting Covid-19, but are expensive and can take up to 48 hours to process in a lab. Other rapid tests identifying Covid antibodies have been developed though few have secured regulatory approval.

Do lateral flow tests have a role to play in ‘living with Covid-19’?

About one in three people with coronavirus don’t have symptoms but can still pass it on to others, so “regular testing of people without symptoms is important to help stop the virus spreading and protect your loved ones,” according to UK government guidance.

However, the review found rapid antigen tests 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. But in people with no symptoms, that drops to 58 per cent. And there were big differences between brands. Little data exists to show how well the tests perform when people including schoolchildren have no symptoms.

The team, however, identify three possible uses for them: testing to detect infection; testing to release people from self-isolation or quarantine; and testing to enable them to go to school, work or an event.

"You can't do such a test for test-to-release or test-to-enable," said Prof Jon Deeks of Birmingham University, one of the study's authors. People would go back into the community and be socially mixing. The tests, he noted, "will probably detect about half the cases".

They create both false negative results - when people are wrongly told they are not infected, and also false positives, when people will unnecessarily be instructed to self-isolate, which could mean they cannot work.

In a mass testing situation, Deeks added, among 10,000 people with a prevalence rate of 0.5 per cent, similar to now in the UK, you would expect 50 people to have Covid. The tests would pick up 35 of the 50, but 90 would wrongly be told they were infected.

With false positives you may end up having the opposite effect of what is sought, ie forcing closure of workplaces or schools.

Where do lateral tests have a valid role to play?

The tests do have a use among people with symptoms, the scientists found. This entails, for instance, targetted use within health services. In settings such as a test centre, a hospital or GP surgery, they can give a rapid diagnosis, which means contacts can be traced faster than at present. But the positive test must then be confirmed with a PCR test.

The UK government maintains the tests are an essential part of the Covid response. In Ireland, Nphet and the HSE in advising the Government have taken a more conservative stance – arguably with scientific justification – in advising the Government on their deployment while acknowledging they have a place in the battle against Covid. They are clearly not good in populations where there is low prevalence of disease, as prediction rates and reliability drops. So they are not going to be the big enabler in opening up societies.

Since last month, however, the HSE has started counting a positive antigen result as an infection in official data; previously, this had to come from a PCR test. Antigen testing has been approved for use in settings such as hospitals, where there are outbreaks and high numbers of symptomatic patients.

Other health specialists have highlighted over reliance on PCR testing when we should be looking at how repeat testing can detect infections in overall populations.

The bottom line remains: antigen tests only deliver a positive result when a person is most infectious, for up to eight days in the infection cycle. Some tests might come up negative, but the person would still be infectious – probably not enough to justify being a key element in a return-to-workplace strategy.

– Additional reporting: Guardian