Officials are investigating a rise in the number of faulty lateral flow tests

Thousands of children with the sniffles may wrongly be being told to stay at home over concerns they have Covid, scientists have suggested. 

Secondary school children are asked to take two lateral flow tests per week. If they test positive, they must stay at home until they receive a result from a gold-standard PCR swab.

Latest Test and Trace (T&T) statistics reveal 18,930 of the 615,000 rapid swabs taken by pupils in the week ending September 29 came back positive. 

Of those, 13 per cent — or 2,053 — were either negative or voided when followed up with a PCR. 

The proportion of children being wrongly told they have Covid by the devices has doubled in a month.

It has prompted health chiefs to launch a probe, with the scientific community now discussing whether the rise is actually down to the resurgence of colds. 

Experts have suggested the heavy mucus samples from children who test positive may be skewing the accuracy of the swabs. 

Gaming tests to get time off school and faulty LFDs have also been suggested as possible factors behind the rise.    

Latest data from Test and Trace shows the proportion of children being told they are positive for Covid when they don’t have the virus by lateral flow devices has doubled in the last month (red line). More than one in ten positive results from lateral flows are incorrect

Latest Test and Trace data showed there were 614,407 lateral flow swabs carried out in the final week of September

Latest Test and Trace data showed there were 614,407 lateral flow swabs carried out in the final week of September

Latest Test and Trace data showed there were 614,407 lateral flow swabs carried out by secondary school students in the final week of September — the most up-to-date data available.

Almost 13 per cent of the positive results were inaccurate (2,053 came back negative or void out of 15,916 positives that were followed up with PCRs).

For comparison, the figure stood at 8 per cent the week before (1,112 out of 13,709).

In the first week of September six per cent of school children (213 out of 3,314) received false positives.

What could be behind the rise in false positives?

More than one in ten pupils who test positive for Covid using lateral flow devices are not infected with the virus, official figures show.

In the latest week some 2,053 secondary school children (13 per cent) who were told they had the virus by lateral flows, tested negative using PCRs. The proportion of false positives has doubled in a month.

Scientists say several factors could be behind the rise. These include: 

Dr Susan Hopkins, the chief medical adviser of the UK Health Security Agency, said: ‘We have been made aware of some areas reporting positive (lateral flow) test results with subsequent negative PCR tests and we are looking into the cause.’

There have been several anecdotal reports of children testing positive with LFDs, but then getting a negative result from PCRs. 

Dr Kit Yates, a mathematician at the University of Bath and member of Independent SAGE, tweeted that officials were saying the false positives were likely down to colds and heavy mucus samples.

‘PHE (a now-defunct body replaced by the UKHSA) advising schools that lateral flow test positives followed by PCR negatives are likely because of colds/heavy mucus samples,’ he said.

‘My understanding is that lateral flow devices are designed to avoid cross-reactivity with other viruses, which suggests this is not the case.’

Writing in The Conversation, Dr Yates said: ‘A number of explanations have been put forward.

‘Some have suggested a faulty batch of lateral flow tests could be causing people to test positive when they don’t have Covid. 

‘Others have hypothesised a new variant could be circulating that isn’t detected by the standard PCR test. 

‘There have also been well-publicised stories of children faking a positive LFT result using the acidic properties of soft drinks. These spoofers would subsequently test negative on a follow-up PCR test.’

He added: ‘But there’s also a potential mathematical explanation, given neither test is 100 per cent reliable.’    

Professor Francois Balloux, a geneticist at University College London and scientific commentator on the pandemic, suggested it was possible some lateral flows were ‘cross-reacting’ with seasonal coronaviruses that cause common colds.

He tweeted: ‘I feel it would be worthwhile investigating further the potential for LFDs cross-reactivity between SARS-CoV-2 (the virus that causes Covid) and endemic HCoVs (other coronaviruses).

‘While somewhat unlikely, this remains a far more plausible hypothesis to me than an increase in PCR false-negatives due to recent SARS-CoV-2 evolution.’ 

Dr Catherine Moore, a clinical scientist for Public Health Wales, suggested the spread of common colds may be behind the rise. 

She said lateral flow tests rely on non-viscous liquids — those which are not thick and sticky — to allow the test fluid to flow across the device.

But in the case of cold and flu nose secretions are more viscous, which stops the fluid flowing as well and could lead to a false positive.

She added that this has been observed with fruit juices, which anecdotal reports suggest could trigger a false positive diagnosis.

Writing on Twitter, Dr Moore said: ‘If the flow is inhibited, you can fail to clean off anything that might cause a false reaction.

‘See all the posts of kids using sugary drinks. The tests rely on non-viscous liquid being applied.’ 

Dr Rupert Beale, a biologist at the Francis Crick Institute, claimed it was still ‘highly speculative’ that a lateral flow could be fooled by a ‘snotty kid’ infected with another coronavirus.

There have been suggestions that other viscous liquids, such as fruit juices, could also trigger a positive test. 

Sir David Spiegelhalter, an eminent statistician at the University of Cambridge, told The Times that some false negatives should also be expected.

‘We know lateral flows have very low false positives, and that PCRs miss infections,’ he said.

Lateral flow tests rely on a nasal swab that is mixed with a solution to diagnose a Covid infection within 30 minutes.

More than £1billion has been spent on purchasing the devices, but studies suggest they may only pick up the virus in the early stages of an infection. And real-world data has shown they may only spot up to 70 per cent of cases. 

On the other hand, PCR tests have been held up as the gold standard for detecting the virus. But they can take around a day to complete.

Government Sources said there is currently no evidence of any technical issues with LFDs or PCRs that could be behind the rise.

They added it was vital to follow the instructions when carrying out a PCR, and that no test is 100 per cent accurate.


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