In Depth

Coronavirus: how do ‘spit tests’ work?

Thousands of volunteers to take part in mass trial of Covid-19 saliva tests from next week

The UK is moving a step closer to nationwide mass testing for Covid-19, as thousands of NHS workers and their families sign up to trial a new weekly saliva test.

Announcing the pilot scheme, which begins next week, Health Secretary Matt Hancock said that “saliva testing could potentially make it easier for people to take coronavirus tests at home”, with no need for swabs. “The trial will also help us learn if routine, at-home testing could pick up cases of the virus earlier,” he added.

More than 14,000 people in Southampton will take part in the first four-week phase of trials, growing to around 40,000 by the end of July. And if successful, the programme will be extended across the country.

Why does it matter?

Regular testing of the whole population, regardless of whether or not they feel unwell, would provide a much more detailed picture of how many people have the new coronavirus and whether it is retreating or spreading.

“It could potentially spot people who are infected but have no symptoms, yet still risk passing the disease on to others,” says the BBC. So-called asymptomatic transmission is otherwise all but impossible to detect, even with effective contact tracing.

According to random tests carried out by the Office for National Statistics, up to 70% of people carrying Covid-19 may be unaware that they are infected.

Early reports of new infection clusters “could be used to deal with local ‘flare-ups’ without the need for extensive lockdowns”, according to The Times.

Keith Godfrey, a Southampton University epidemiology professor working on the trial, says the initiative can “contribute to safely restoring economic activity within the city and region”.

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Health officials also hope that the new saliva tests will replace the “invasive, and sometimes painful, deep nasal and throat swab”, reports ITV News

Inserting the swab “provokes coughing and spluttering, putting health workers - already working in close quarters with the testee - at even greater risk of the droplet-borne infection”, the broadcaster says. 

The existing tests are also “believed to return false negative results in up to 30% of cases”, adds The Times.

How do the tests work?

“Saliva tests have looked promising for a while” but until now have not been widely used, says The Guardian.

The new test, made by West Sussex-based biomedical firm OptiGene, “looks for genetic material of the virus using a technique known as loop-mediated isothermal amplification”, explains the BBC.

Swab tests rely on a different testing system, which requires expensive lab equipment to raise and lower the temperature of the sample, encouraging the virus to grow to detectable levels. 

By contrast, saliva test samples can be kept at a constant temperature throughout the assessment process, which is relatively simple, quick and cheap. 

The new system has “an added advantage because saliva preserves the virus, so there is less urgency than with swabs, which have to be rushed to the lab”, says The Guardian.

How will the Southampton trial work?

Participants will spit into a plastic tube, “which they will either send off or have collected by Southampton University staff running the trial”, The Guardian reports. “They should get a result within 48 hours.”

Details of those who test positive will be sent to NHS contact tracers, who will notify anyone else who may have been infected.

“Ultimately, it might be possible to do the testing as well as the sampling at home and get results in under an hour,” says the BBC.

During the trial, the saliva tests “will be validated by comparing the results with traditional swab tests”, says The Times. “It will eventually include 40,000 people and provide the first concrete evidence of whether mass testing can be effective at scale in reducing infection rates.”

Who is eligible to enrol?

The first people invited to join will be staff in GP surgeries and their families, who will start weekly testing next week. 

Over the following four weeks, “other essential key workers, university employees and their families” will also be invited to sign up, says The Guardian.

Swab tests will still be used in routine testing for NHS staff and anyone with symptoms who requests a test.

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