Almost a third of negative coronavirus test results could be wrong, scientists say, leaving thousands believing they are virus free.
People who are wrongly told they don’t have the virus when in fact they do – called a ‘false negative’ – could be spreading the disease if they think they are safe to return to work.
False negative results would mainly be the fault of incorrect swabbing, experts say. Health chiefs have also admitted the test itself, called a PCR test, is ‘not perfect’.
Health workers are trained on how to swab a potentially infected person, while home kits come with instructions – but experts say there are bound to be mistakes.
Scientists say it is ‘dangerous’ to rely on test results to solely steer the handling of the pandemic, in which at least 34,466 Britons have died.
They argue symptoms should not be brushed aside just because a person has a negative result and they should be told to self isolate just in case.
It follows warnings to ministers that up to one in four COVID-19 cases will be missed because the symptom list is not broad enough.
Health chiefs in the UK told the public to look out for a cough and high temperature for months.
But by missing out aching muscles, loss of taste and smell and headaches, experts fear Britain will not get a grip on the crisis.
Health workers are trained on how to swab a potentially infected person, while home kits come with instructions – but experts say there are bound to be mistakes. Pictured: A health worker takes a swab to test a key worker at Royal Papworth Hospital in Cambridge on May 5, 2020
Nasopharyngeal swabs are the preferred choice of testing for SARS-CoV-2 worldwide because it collects the most concentrated sample. A long flexible cotton bud is supposed to be inserted deep into the nostril and along the nose ‘floor’ to collect a mucus sample. But the invasive test is so uncomfortable it’s been described like ‘being stabbed in the brain’
There have been almost 2.5million tests across the UK so far, according to the Department of Health, of which 240,161 have been positive.
Some 40 per cent of tests have been repeat tests to clarify if someone has got rid of the virus, for example when a patient leaves hospital.
Public Health England have not revealed how many test results could be incorrect.
But experts believe false negatives are in the region of 10 to 30 per cent.
Lawrence Young, a virologist and infectious disease expert at Warwick University told MailOnline: ‘I’m guessing around 30 per cent of results are a false negative.
‘That means for every 100 people you test that are positive, 30 will come out as negative.’
Tens of thousands more people are having swab tests now than at the start of the outbreak as the Government ramps up capacity.
Despite the surge in test numbers, ministers have boasted that daily new cases are levelling off as a sign the outbreak is slowing.
But Professor Young asks: ‘Are we accounting for up to 30 per cent of samples we are testing that is negative is actually a positive?
Tens of thousands more people are having swab tests now than at the start of the outbreak as the Government ramps up capacity (see top right). Despite the surge in test numbers, ministers have boasted that daily new cases are levelling off (bottom right) as a sign the outbreak is slowing
WHAT IS THE SWAB TEST FOR CORONAVIRUS?
Nasopharyngeal swabs are used to detect respiratory viruses, such as the flu and the new coronavirus.
It is the preferred choice for SARS-CoV-2 testing, according to the Centers for Disease Control and Prevention (CDC).
It involves inserting a long, flexible cotton bud into the nostril and along the nose ‘floor’. This is supposed to be done slowly so that it is comfortable.
The aim is to reach the posterior nasopharynx, a cavity made up of muscle and connective tissue, covered in cells and mucous that are similar to the nose. It continues down into the throat.
The swab is rotated several times in order to get enough cells.
The sample is then sent to a lab, where it will be tested to determine if the patient’s cells are infected with the virus.
The coronavirus is a RNA virus, which means it uses ribonucleic acid as its genetic material. A process called reverse transcription is needed to transcribe the RNA into readable DNA.
A swab sample doesn’t collect much RNA in one go, therefore a polymerase chain reaction (PCR) is used to rapidly make billions of copies so it can be analysed.
The DNA is dyed a fluorescent colour, which glows if the coronavirus is present, confirming a diagnosis.
‘False negatives are for several reasons, but probably the main reason is a sampling error. It’s always concerned me and many colleagues there is an issue with swabbing and I think some errors will creep in.’
Nasopharyngeal swabs are the preferred choice of testing for SARS-CoV-2 worldwide because it collects the most concentrated sample.
A long flexible cotton bud is supposed to be inserted deep into the nostril and along the nose ‘floor’ to collect a mucus sample.
The aim is to reach the posterior nasopharynx, a cavity made up of muscle and connective tissue, covered in cells and mucous. It continues down into the throat.
But the invasive test is so uncomfortable it’s been described like ‘being stabbed in the brain’. It can cause people to gag and suffer nosebleeds.
The alternative is to take two swabs; one from the nostril and another of the back of the throat through the mouth.
Home testing kits – of which some 35,000 are currently being sent out per day to key workers and their household members – use this method.
Studies from early in the outbreak in China have suggested that test results vary depending on the type and the day the swab is taken in the course of the disease.
One study, published on MedRxiv, showed nasal swabs correctly identified only 73 per cent of severe COVID-19 cases and 72 per cent of mild cases in the first week of symptoms.
The positive rate of throat swabs was even lower, around 60 per cent. When throat swabs were collected more than eight days after symptoms, the positive rate dropped to 50 per cent in severe and 29.6 per cent in mild cases.
Eight days may seem like a long wait for a test. But in the UK, accessing tests has shown to be challenging. Home test kits and slots at facilities have been selling out within minutes online.
Professor Young, who emphasised that nasopharyngeal swabs collect a stronger sample, said he believes there is an ‘enormous variability’ in how swabs are being collected, both at home and at drive-through testing facilities across the UK.
‘You have to go deep into the nose. When you look at some pictures [of testing facilities], that’s not happening,’ he warned.
‘If you’re going to make judgements on one swab test, you have to be very careful especially if you are telling people if they can go back to work.’
Paul Hunter, a professor in medicine and infectious disease scientist at University of East Anglia said relying on test results is ‘dangerous’.
Home testing kits – of which some 35,000 are currently being sent out per day to key workers and their household members – tell people to take two swabs; one from the nostril and another of the back of the throat through the mouth
Professor Young said he believes there is an ‘enormous variability’ in how swabs are being collected, both at home and at drive-through testing facilities across the UK. Pictured: A drive-in testing facility at the Chessington World of Adventures Resort, west London
He told MailOnline: ‘What I’m worried about is if you’ve got symptoms, and the test comes back as negative, you can go out. That is dangerous because you get a lot of false negatives with the PCR test.
‘I think it’s dangerous, and it will get more dangerous, if the government continues to rely on test results as cases decline. Because a wrong result could lead to wider spread, particularly if the person is a super spreader.
Professor Hunter said he ‘believes clinical symptoms as well as the test result are important in assessing risk’.
Similarly, Nick Summerton, a GP in East Yorkshire, has raised concerns that people who tested negative were not being sufficiently warned that they might have the disease.
He told The Sunday Times: ‘On most days I come across a person who has “barn door” COVID-19 symptoms but a negative result.
‘They often know as well as I do that this must be a false negative, but the post-test guidance is pretty useless in helping them with this.’
The most common symptoms of the virus are a persistent cough and high temperature. These are the only signs listed by the NHS and are what Public Health England insists a person must have in order to access a test.
Dr Summerton, who has also acted as an advisor to the Government during the pandemic, has called for symptoms such as tiredness, breathlessness and loss of appetite to be included in key symptom list for Britain.
He said the public may be reassured they don’t have the coronavirus if they look at the NHS website, therefore unwillingly go around spreading it to others.
He warned the UK will fail to get ‘a grip on the epidemic’ by ignoring the presence of symptoms which may have previously been seen as uncommon, The Sunday Times reported last week.
Compared with adults, children are less likely to have the tell-tale symptoms and may not show any signs of the virus at all. Pictured: Around 73 per cent of US children have a fever, cough or shortness of breath, compared with 93 per cent of adults
WHAT SYMPTOMS DO HEALTH OFFICIALS LIST? CDC’S PREVIOUS AND CURRENT LIST OF CORONAVIRUS SYMPTOMS
The NHS lists the following as the main symptoms of the coronavirus:
- a high temperature – this means you feel hot to touch on your chest or back (you do not need to measure your temperature)
- a new, continuous cough – this means coughing a lot for more than an hour, or 3 or more coughing episodes in 24 hours (if you usually have a cough, it may be worse than usual)
It also says to use the NHS 111 service if you have those symptoms
The Centres for Disease Control and Prevention (CDC)
Until recently, the CDC only listed three symptoms of coronavirus on its website:
- Shortness of breath
Last week, the CDC expanded its list to include the following signs:
- Repeated shaking with chills
- Muscle pain
- Sore throat
- New loss of taste or smell
The World Health Organization
Most common symptoms:
- Dry cough
Less common symptoms:
- Aches and pains
- Sore throat
- Loss of taste or smell
- A rash on skin, or discolouration of fingers or toe
- Difficulty breathing or shortness of breath
- Chest pain or pressure
- Loss of speech or movement
Ministers have been warned that the virus may be undetected in a quarter of cases because they exhibit symptoms of the virus other than a cough and fever, The Telegraph reports.
Scientists on the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) expressed concern that people with only lesser known symptoms may not be self isolating or getting access to a test.
They raised issues at a Nervtag meeting on March 13, the day after Prime Minister Boris Johnson first told the public to self isolate if they developed a cough or high temperature.
According to minutes of the meeting, John Edmunds, professor of infectious disease modelling at the London School of Hygiene & Tropical Medicine, pointed towards a paper that said ‘one in four cases could be missed if the criteria of only a cough and/or fever are used’.
The minutes of the meeting said: ‘There was concern that the phraseology might mean that a number of individuals with the virus may not meet the definition.’
Research has pointed towards an array of symptoms SARS-CoV-2 can cause beyond the typical cough and fever. Sometimes these depend on the severity of disease.
On March 31, King’s College London researchers said almost 60 per cent of coronavirus patients experienced a loss of taste and smell.
They collected data of more than 1.5million Britons who downloaded the COVID Symptom Tracker App.
Some 59 per cent of those who reported their health by March 29 and tested positive reported a loss of smell and taste, compared with 18 per cent of those who tested negative.
But the Government discarded loss of smell and taste – which Health Secretary Matt Hancock said he suffered with during his illness – as ‘anecdotal evidence’ on April 3.
The World Health Organization lists 13 symptoms of COVID-19 and the CDC bumped its up to nine in recent weeks.
Both acknowledge muscle pain, loss of taste and smell and headaches as typical signs of SARS-CoV-2 infection.
Peter Openshaw, a professor of experimental medicine at Imperial College London, who sits on Nervtag, said it was important patients and doctors broadened their perception of how the coronavirus presents itself in different people.
He said: ‘In the self reported symptoms, the loss of sense of smell comes out as a very, very frequent one.
‘But in terms of how often that would actually lead to a diagnosis of COVID, that’s not quite clear.
‘The original, very restrictive definition, I think, was not picking up a lot of people who actually might have COVID.’
Professor Openshaw confirmed that one in four cases could still ‘easily’ be missed as a result of the focus on coughing and high temperatures.
MailOnline have contacted The Department of Health and Public Health England for comment.
Yvonne Doyle, Medical Director at Public Health England said: ‘The UK testing system is built on strong foundations using the latest evidence and expert advice. There are many studies currently ongoing into the accuracy of the COVID-19 test and a number of different methods are available to assess this. However, there is no such thing as a perfect test.
‘We have made every effort to make testing as strong as possible in line with our high standards.’