Coronavirus: One in 10 positive lateral flow tests in England turn out wrong when people retest

At least one in 10 positive results given by rapid lateral flow coronavirus tests were wrong in March, Public Health England data show.

Since self-testing kits were put into mass use when schools in England reopened last month, 3,248 positive results have been found to be false by follow-up swab tests.

The controversial kits, which have irked scientists who warn they are less accurate when people swab themselves, are now available for free to all adults with no Covid symptoms.

But somewhere between 10 and 20 per cent of the positive results they give could be wrong, meaning people end up panicked and sent home from school or work unnecessarily.

Between March 8 and April 7 — when the 3,248 cases were found, 31,000 test results from lateral flow kits came back positive.

In total, more than 8,000 positive results from the tests have been wrong since they started being used before schools reopened.

Government officials insist the benefits of testing people without Covid symptoms – weeding out silent carriers of the virus – far outweigh their pitfalls.

One outspoken critic of the self-testing policy, Professor Jon Deeks, said that tens or hundreds of people could be affected by one person getting a false positive result.

He added: ‘If you get a positive result it is essential that it is confirmed by a PCR test as soon as possible to check that you and your contacts do not isolate unnecessarily.’

Rapid self-testing kits are now used by schools and employers across the UK. Public Health England has reported at least one in 10 positive results are found to be wrong by follow-up lab tests (Pictured: A young woman tests herself at the University of Hull)

The controversial kits, which irked scientists because they are less accurate when people do the swabs themselves, are now used millions of times a week in the UK. The tests work by someone swabbing their throat and nose, mixing the swab in a testing liquid and then pouring the liquid onto a plastic cassette that gives a result in 15 minutes

A total of 3,248 positive results counted in the Department of Health’s daily updates were later scratched from the record for the period March 8 to April 7.

These were results that were positive when someone took a lateral flow test but then negative when they took a better quality lab test afterwards.

They came from a total of 30,956 rapid test positives recorded in that time — out of 29.25million tests taken.

This means roughly 11 per cent of all positive results from lateral flow tests – hailed as being key to restarting Britain’s crippled economy – turned out to be wrong.

And the figure may be even higher because only half of the rapid tests (15,349) were actually followed up with a lab swab.

Professor Deeks, a medical testing expert at the University of Birmingham, told MailOnline: ‘For schools, a child testing positive will mean themselves, their family, their contacts, their childcare all being affected for 10 days.

DATA SHOW OVERALL FALSE POSITIVE RATE IS EXTREMELY LOW

Although the Public Health England data show thousands of people have been wrongly told they have coronavirus, it also suggests that the false positive rate of the test is extremely low.

During the period from March 8 to April 7, only 0.01 per cent of people who took a rapid test got a positive result and then tested negative with a PCR test.

Out of 30,956 positive results from the lateral flow tests, 3,248 got negative results from a later lab-based test.

While this is a rate of over 10 per cent, the test correctly detected millions of other negative results at the same time.

Overall, 29,251,374 people took a rapid test in that time frame.

The proven false negative rate, therefore, was only 0.01 per cent, suggesting the test successfully detects 99.9 per cent of negative results.

The true specificity of the test – its ability to give the right result to someone without the virus – cannot be calculated without knowing exactly how many people should be negative, but this can act as a rough guide.

‘Certainly tens and possibly 100 people. If it was a teacher it could be worse.’

In February and March, positive rapid tests were not all followed up with a lab test to confirm the result.

Although this policy has now been reversed and all people who get a positive result are told to take a follow-up test.

The data suggest that the overall rate of false positive rates is extremely low – over 99.9 per cent of the tests appear to give a negative result or a true positive.

Professor Deeks said around two in every 10,000 tests could be a false positive, based on data so far.

But because so many are used in Britain, even the small margin of error has the potential to affect a lot of people.

In the last week of the school term before Easter, an average of 810,488 rapid tests were taken every day, out of which 160 could have been false positives.

This is why it’s vital for follow up positive results with a second, higher quality test.

Lab-based PCR tests are considered the gold standard but they are more expensive and can take up to three days to produce a result.

Professor Deeks said acting on the results of lateral flow tests alone was ‘exactly the wrong thing to do’.

People should not act as if they are virus-free if they get a negative result, he said, and it was also not ideal to start full contact tracing if the result is positive.

People should, however, isolate and get a proper PCR test if their rapid result is positive.

‘It is a mathematical certainty that as disease rates go down, the chances go up that a positive result from these lateral flow tests will be false,’ Professor Deeks added.

‘If you get a positive result it is essential that it is confirmed by a PCR test as soon as possible to check that you and your contacts do not isolate unnecessarily.

FREE RAPID TESTS AVAILABLE TO EVERYONE IN THE UK

England’s Covid testing blitz began last week, with everyone in the country now able to use self-test swab kits twice a week with the aim of getting life back to normal.

Boots and local pharmacies are now offering lateral flow tests that adults can go in and pick up for free, even if they don’t have any symptoms of the virus.

Matt Hancock credited the scheme as being ‘one of our most effective weapons in tackling this virus’ and the Health Secretary said the campaign would cut the spread of asymptomatic Covid, which is a huge driver of the pandemic.

Ministers say the programme is an essential part of the easing of restrictions, helping identify variants and thwarting outbreaks.

But critics say the rapid kits could wrongly label tens of thousands of people a week as having Covid when used at such large scale — muddying the water about whether the disease is making a comeback.

The lateral flow kits available on the new scheme are free of charge and dished out at approved testing sites, more than 5,000 pharmacies and by post. People can collect tests for up to four household members at a time.

The tests can be ordered through the government website – link.

‘There is still a problem that this test will miss many cases. The Government has done too little to evaluate this test in people without symptoms before launching the biggest testing campaign in our nation’s history.’

The Government still defends the use of the kits, saying the tests save lives by spotting Covid cases ‘we wouldn’t otherwise know about’.

A Department of Health spokesperson said: ‘Around one in three people with coronavirus do not show any symptoms.

‘So every day rapid testing is helping us find cases of Covid we wouldn’t otherwise know about, breaking chains of transmission and saving lives.

‘Lateral flow tests are convenient and reliable with the vast majority of positives correctly indicating infectiousness.’

Everyone in Britain can now take two lateral flow tests a week for free, regardless of whether they have any symptoms. They will get results within as little as 15 minutes.

Ministers hope the multi-billion-pound swabbing blitz – which involves Boots and other local pharmacies – will help life revert back to normal.

But scientists have warned the scheme could result in a ‘public health disaster’, with thousands forced needlessly to self-isolate and others given false confidence that they are safe.

One of the biggest issues is that the tests have low sensitivity, meaning they do not pick up everyone who is carrying the virus.

People who have the virus but still test negative – false negatives – might get misplaced confidence from the test result and be less strict about following lockdown rules that someone who hasn’t been tested, experts say.

This could lead to them going back out into the community and spreading the virus without knowing.

But government experts are satisfied the DIY swabs, widely used by schools, care homes and the NHS, are a key tool in reopening society.

The tests are said to already have identified 120,000 cases that might not otherwise have been picked up.

LATERAL FLOW TESTS ARE LESS RELIABLE WHEN PEOPLE SWAB THEMSELVES

Lateral flow tests are only accurate at diagnosing coronavirus when administered by trained professionals, studies have repeatedly shown.

The tests, which give results in as little as 15 minutes, use swabs of the nose or throat. Samples are then mixed in a testing liquid and put into a plastic cassette which can detect the presence or absence of coronavirus and then produce an image of a line, the same way as a pregnancy test, to indicate whether it is positive or negative.

The Department of Health and NHS are instructing people to use the tests on themselves, despite manufacturers of some kits saying they shouldn’t be used as DIY swabs.

Both the swabbing procedure and the use of the test cassette can easily be done wrong and affect the accuracy of the test.

If the swab isn’t done for long enough, or deep enough into the nose or throat, it may not pick up fragments of virus. Medical professionals are also able to use nasopharyngeal swabs, which go right to the back of the nostril, whereas this is not advised for people who test themselves.

And if the sample isn’t properly inserted into the cassette the result might be wrong, or people may misread the display when it produces a result.

SELF-TESTING CUT ACCURACY FROM 79% TO 58%

A University of Oxford and Public Health England evaluation of the Innova lateral flow test, which is being widely used in the UK, found its sensitivity – the proportion of positive cases it detected – fell from 79 per cent to 58 per cent when it was used by untrained members of the public instead of lab experts.

Based on this evaluation, officials pushed ahead and used it for a real-world self-testing trial.

PILOT IN LIVERPOOL FOUND FEWER THAN HALF OF POSITIVES

When the same Innova test was trialled on members of the public in Liverpool – with people taking their own swabs and trained military staff operating the tests – the swabs picked up just 41 per cent of positive cases.

In the study the rapid tests detected 891 positive results, compared to lab-based PCR swabs that found 2,829 positives in the same group. This means 1,938 people got a wrong negative result from the rapid test.

The study didn’t compare this to professionally done rapid tests, but the manufacturer Innova claims its test is 95 per cent sensitive in lab conditions.

…BUT TESTING DONE BY MEDICS IN SLOVAKIA ‘REDUCED INFECTIONS’

Despite rapid lateral flow tests getting bad press, officials in Slovakia used them on 5.2million people – almost the entire population of 5.5m – in a trial that a study later estimated to have cut the country’s infection rate by 60 per cent.

The tests used were between 70 and 90 per cent accurate and all the swabs and evaluations were carried out by trained medical workers. They used deep nasopharyngeal swabs, that go to the back of the nose, whereas self-testing generally relies on a swab of only the nostril.

London School of Hygiene & Tropical Medicine researchers said that the scheme successfully weeded out coronavirus cases that wouldn’t have been found otherwise, slashing the number of cases by over half in a week during a lockdown.

HOW RAPID TESTS ARE DIFFERENT TO LAB-BASED PCR SWABS

Lateral flow tests are an alternative to the gold standard PCR test – known scientifically as polymerase chain reaction testing – which is more expensive and more labour-intensive but more accurate.

PCR tests also use a swab but this is then processed using high-tech laboratory equipment to analyse the genetic sequence of the sample to see if any of it matches the genes of coronavirus.

This is a much more long-winded and expensive process, involving multiple types of trained staff, and the analysis process can take hours, with the whole process from swab to someone receiving their result taking days.

It is significantly more accurate, however. In ideal conditions the tests are almost 100 per cent accurate at spotting the virus, although this may be more like 70 per cent in the real world.

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