Covid: Rapid tests 'useful public health tool'

Rapid tests for coronavirus are a "useful" public health tool despite only picking up half as much virus as tests done in a lab, a group of scientists has said.

The 30-minute test has been criticised for how it measures up to the tests processed in the government's mega-labs.

But the scientists cautioned against comparing them directly.

The tests serve different purposes in different contexts, they explained.

And rapid tests may be a better guide to who is currently infectious.

During a testing pilot in Liverpool, the devices picked up two-thirds of the most infectious cases - all people who would not have otherwise been tested at all.

The pilot, which made tests available to anyone in the city who wanted one, picked up 1,300 infections in people without symptoms in a population of about half a million.

Prof Louise Kenny at the University of Liverpool said even if one in three infections was missed, that left two infections which would not otherwise have been found.

No test was perfect, she said, but she warned against "making perfection the enemy of the good".

However, the evaluation of the Liverpool pilot found there was "no clear evidence that the introduction of 'mass' testing in Liverpool impacted on Covid-19 cases or hospital admissions."

The scientists on the panel included those involved in evaluating rapid tests in Liverpool, Oxford and the Porton Down research facility.

Pros and cons of PCR and lateral flow

Standard lab tests, also known as PCR tests, pick up some infections that rapid or "lateral flow" tests miss, particularly in the early stages of infection.

These are the tests you'll have if you have symptoms.

PCR tests are very important for use in diagnosing people - by the time someone turns up at a hospital they may not have much virus in their nose or throat, as it migrates to the lungs. So in these situations having a very sensitive test is useful.

They are crucial for use in surveillance of the virus, and they're also important for contact-tracing since, even if that individual is no longer infectious, they may have passed it on to others who are.

But the downside is they pick up small fragments of inactive virus, meaning some people might be told to isolate when they are no longer capable of infecting anyone.

Rapid or lateral flow tests, on the other hand, are not good at picking up relatively small amounts of virus.

Overall, the Liverpool study found the devices only picked up 40% of cases - less than expected.

This means the tests will miss people in the day or two before they are about to become infectious, as well as those who have cleared the infection from their systems.

But they do find the majority of the most infectious cases - and lateral flow tests have the major advantage of not needing to be taken to a lab.

They work by dipping a nose and throat swab in special solution and then dropping that fluid onto a plastic stick a bit like a pregnancy test.

"The time and scale gained by having a low-cost test that returns results within an hour without a laboratory is a valuable new tool for tackling Covid-19," according to Prof Iain Buchan, who led the Liverpool evaluation.

This allows for many more people to access testing, including those without symptoms - people for whom it is "this or nothing", Prof Kenny said.

Context is key

Screening asymptomatic people in the community is potentially nothing but positive, provided people don't change their behaviour after a negative result - and isolate after a positive one.

It's a more difficult question once you are using them to clear people to do something - for example visit a care home resident or cross the border into France.

Missing perhaps a third of infections - although it can be less if the test is done by a trained person - might be too high a risk in those cases.

But even if you are allowing someone to do something they wouldn't have otherwise, the tests can allow people to balance risk with harm.

They might provide reassurance to allow someone in a care home who has been deprived of social contact to have a visitor wearing PPE.

Or, if there is one Covid case in a school year, it could be used to prevent a whole class from having to isolate and miss face-to-face education.

They don't eliminate risk, but they can reduce it in situations where there is enough benefit.

And their accuracy can be improved by screening people regularly, Prof Kenny explained - perhaps once or twice a week in schools, universities and specific workplaces.

Prof Jacqui Ramagge, who led student testing efforts at the University of Durham, said "the chances of someone being positive when they are repeatedly testing negative is very, very low".

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