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People taking coronavirus tests may get a negative result back – but that doesn’t necessarily mean they’re clear of the virus.

Reports suggest tests are coming back with “false negatives”, where people are effectively told they don’t have Covid-19, despite continuing to struggle with symptoms. 

The main type of test used in the UK – and around the world – is the polymerase chain reaction (PCR) test, where a sample is taken by swabbing the nose and/or throat, and then tested in laboratories. People can have this in hospitals, as well as via drive-through centres and home testing kits.

So how accurate are they – and what could cause a “false negative” result? 

What do we know about testing accuracy?

Different studies have shown varying levels of accuracy for the PCR tests – and experts don’t agree on how accurate they are at this stage.

Professor Lawrence Young, a virologist and expert in molecular oncology from Warwick Medical School, tells HuffPost UK: “We estimate that, at best, [the test is] 75% accurate. That is, for every 100 people who are tested who are actually infected, you’ll miss probably, on average, 25 people and they will come out as false negatives.”

With the UK government aiming to carry out 100,000 tests a day at this point, it could mean as many as 25,000 are coming back with a false result.

But the accuracy is improving, says Dr James Gill, a GP locum and honorary clinical lecturer at the University of Warwick. Where the test once had about a 20-30% false negative rate, he believes it’s now nearer to around 5-10%. 

Other experts HuffPost UK asked were unable to put a figure on the accuracy of these tests. Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, says: “The reason I – and many others – won’t cite specific accuracy data is there are so many studies [looking into this], and the methods used are often different, and this makes a big difference to the number,” he explains.

“I can’t say, ‘This test is 80% accurate and that test is 95% accurate’ – it’s far more complicated than a single number.”

A study from China back in February, which compared chest CT scans and RT-PCR tests, concluded that chest CTs “had higher sensitivity for diagnosis of Covid-19”. Of the 1,014 patients tested, 59% had positive RT-PCR results, and 88% had positive chest CT scans.

Among the patients whose RT-PCR results came back negative, 75% then showed positive on the chest CT scans.

What could cause false negative results?

Prof Young believes sampling error plays a part. “The way you take the sample with swabs in the nose and throat is very inaccurate – and indeed, having seen some of this on the TV where people are driving into these testing centres and you see the swabs being taken, I’m very concerned they’re not actually swabbing correctly,” he says. Dr Gill agrees: “It’s a swab that requires some experience to do well, certainly without being uncomfortable.”

At the moment, the tests in widespread use rely on three things: the right sampling, extracting the sample once you’ve got it in the right way, and a correct analysis of it in laboratories. A more accurate sample could be taken in a person’s saliva, Prof Young suggests, rather than their nose and throat.

There’s also a biological issue that may cause false negatives. When you sign up for a test, you’re told to take it within three days of developing symptoms. This is because the virus moves around your body. In the early days of developing the virus, it might stick around in your nasal passage or the back of your throat, but as the days pass, it will begin to move towards your lungs.

“The longer you have the condition, the virus migrates and it goes down into your lungs,” says Dr Gill. “So even though you’re coughing, you’ve got the fever, we might not be able to get the swab because it might not be there anymore. It might’ve gone down to the lungs. That’s one reason we can miss it.”

Another factor causing false negative test results is very simply that you can test negative one day, then pick up the infection, and a few days later test positive.

There are many ways diagnostic tests can give incorrect results, says Prof. Edwards, but by no means does this mean the testing service is bad. “I’d say the UK testing system is built on very solid foundations,” he says. “We have strong systems in place to make the tests as good as possible. We have very high standards. But there are fundamental barriers to ‘perfection’.

“Although no tests are perfect, they are still most often correct, and incredibly important and useful.”

What to do now?

NHS England said in a webinar that GPs and other NHS staff who have tested negative for Covid-19 should not return to work if they still have symptoms, Pulse Today reported.

Meanwhile a spokesperson for Public Health England (PHE), which is responsible for testing in hospitals, tells HuffPost UK: “No test is 100% sensitive. False negatives may occur occasionally and there are a number of factors that can affect the result of PCR tests, including the quality and type of sample taken, the timing of the sample in relation to the illness as well as the test performance. Like any diagnostic test, results should be interpreted in the context of the clinical presentation.”

If you’re unsure whether to go back to work after receiving a negative result, it’s a matter of listening to your body. “If you’ve got a temperature, you’re coughing, you’re feeling unwell, you’ve lost your sense of smell and a bit of paper says ‘You’ve not got it’, how much weight are you going to put on that?” says Dr Gill.

Giving a real-world example, Dr Gill explains: “We had a consultant in one of the ITU [intensive treatment unit] departments at the start who developed a cough, and somebody said: ‘Well, let’s go and get you tested.’ He said: ‘No, I’ve got to be off for 14 days.’ And they said: ‘No, we need you.’ He said: ‘Yes, but what happens if the test comes back negative? I could take out the entire ITU department.’”

Prof Young and Prof Edwards believe we need to carry out more than one test on people with suspected coronavirus to get a more accurate picture.

“The bottom line is it’s not 100% accurate and you could argue well, most tests aren’t,” says Prof Young. “But if we’re going to rely on it to make decisions about who should go back into the frontline or who should come out of lockdown or isolation, then the only approach is to have more than one test.”

If you’re still coughing and have a temperature after a test result comes back negative, speak to your GP. The advice will likely be that you need to stay at home and continue self-isolating. “We will always be informed by the results we’re getting,” adds Dr Gill, “but we will treat the person in front of us.”

HuffPost UK has contacted the Department of Health and Social Care (DHSC) for comment and will update this piece when we receive a response.