If lateral flows are 'so accurate', why are so many reporting confusing results?

What is the most accurate test for Covid? It’s a simple question, but while new research suggests that lateral flow tests (LFTs) are more accurate than previously thought, social media is awash with reports of inaccurate results.

The new peer-reviewed study shows LFTs detect more than 80 per cent of infections – regardless of whether people have any symptoms.

Yet anecdotally many are finding the tests unreliable. As veteran broadcaster Mark Austin tweeted: “How come I tested positive on a PCR and throughout the next 10 days every lateral flow I did was negative?”

Bournemouth mum, Hannah responded, “We’ve got one kid who has never tested positive on an LFT but tested positive on a PCR and the other who has consistently tested positive on LFT since Sunday and 2 negative PCRs.”

Meanwhile, Jo Woodward from Liverpool complained: “Other way round for my son, 3 negative lateral flow tests but had the sense to still get a PCR, which was positive.”

It’s a confusing picture, but the lead author of the new study, Professor Irene Petersen, from the Institute of Epidemiology and Health Care at University College London, believes the fact that LFT detects infections without symptoms makes them the most effective for reducing infections.

But trying to compare LFTs and PCR tests is “like trying to compare apples and pears”, she says.

So how can we make sense of this new research and so many reports of false positives and negatives? Petersen says the key is to understand how the tests work, and what they’re designed to detect.

What is the difference between a LTF and a PCR test?

A PCR test identifies genetic material from the Covid virus and is so exquisitely sensitive it will show a positive result long after someone is capable of spreading the virus. This makes it ideal for confirming a diagnosis once someone has developed symptoms.

PCR tests are considered the ‘gold standard’ diagnostic test, with only 3.5 per cent returning a false negative result. However, as PCR tests look for fragments of ribonucleic acid (RNA) which are left after an infection, they are no good at identifying people who are actually infectious but not yet showing symptoms – who are thought to account for a third of transmission.

LFTs respond to a protein on the surface of the virus and can detect signs of infection before any symptoms emerge. Unlike PCR tests, they don’t require processing in a laboratory and the results are available in around 20 minutes.

“They are much more likely to identify people who are actually infectious,” says Petersen.

How can someone have Covid symptoms and a negative LFT?

Professor Alex Richter of Birmingham University explains: “It is impossible to tell the difference between a cold and Covid clinically.” And with ‘the worst cold ever’ sweeping across the UK, thousands of people are experiencing Covid-like symptoms who don’t actually have Covid.

And as Professor Alexander Edwards, from the School of Pharmacy at Reading University, points out: “It’s quite possible for a swab to ‘miss’ collecting any virus, so false negatives can happen for PCR and lateral flow.”

This is most likely to happen if someone doesn’t insert the test swab deeply enough into the nasal cavity, so they don’t actually collect any virus. As Petersen says: “It’s very uncomfortable. I don’t think anyone enjoys taking a test.”

One study reported that only 44 per cent of the YouTube videos showing how to do a nasal swab showed the correct technique.

As Mark Austin asked, how can someone have a negative LFT and a positive PCR?

Petersen says: “PCR tests are exquisitely sensitive, so they detect genetic material from the Covid virus long after people have cleared the infection. Only 50 per cent of people who test positive with PCR are actually infectious.”

How is it possible to test positive on LFT and negative on PCR?

In theory, this is highly unlikely, but there have been a number of reports of this happening around Bristol and the South West. Professor Kit Yates, a mathematical biologist and member of the Independent Sage committee, raised the alarm when he tweeted: “Many of us are hearing a lot anecdotally about people testing positive on multiple lateral flow tests but then negative on multiple PCR tests.”

Jamie Bryce, 41, from Chippenham, Wiltshire, and his son Hamish, 12, tested positive on three lateral flow tests, but negative on PCR tests.

Yates says: “There is definitely a statistical anomaly. The most likely explanation is a faulty batch of tests.”

Are there times when it is more likely to get a misleading test?

Because PCR tests pick up fragments of the virus weeks or even months after someone has been infected with Covid, when infection rates have been high for some time there will be a large pool of people who test positive but are not infectious.

How can I have Covid symptoms, but it can take days to test positive with a LFT test?

You may develop symptoms but still have a relatively low viral load – so there might not be enough virus for a positive result.

If you don’t push the swab high into both sides of the nasal cavity and turn it around you might not collect enough of the virus to get an accurate result. Not blowing your nose properly before the test also increases the chance of a false negative as the swab will collect more mucus than material from the lining of the nose.

There’s also the possibility that a cold could be followed by Covid. After almost two years of social distancing and increased hand-washing, we have had far less exposure to seasonal cold viruses – which means we also have less resistance to them.

When should I take a test?

Petersen advises taking a LFT if you think you may have been in contact with someone who has Covid, or you are planning to visit someone who is vulnerable. “Apart from vaccination, lateral flow tests are one of the most powerful tools we have for stopping the infection.”

As one in three people with Covid have no symptoms at all, she suggests that anyone who comes into contact with a lot of people should take a LFT every three or four days, and to isolate if they get a positive result – regardless of whether they have symptoms.

If you get a positive result on a LFT you should isolate and take a PCR. But if you have symptoms, a positive LFT, but a negative PCR, you should assume you have Covid.