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Are lateral flow tests accurate and how long are the results valid?

There has been a huge demand for lateral flow tests in the UK in recent weeks, following the spread of the Covid Omicron variant across the country.

New guidance has suggested that the latest variant is so infectious that people heading out to potentially high-risk situations should do a test right before they go, instead of within a day or two of the event.

With so many tests now being used, people have been left to wonder if they actually show an accurate result – or whether they should be taking a PCR test also.

Just how accurate are lateral flow tests – and how long do the results last?

How accurate are lateral flow tests?

According to a study from UCL, Liverpool University, Bath University and Harvard, which was published in October 2021, lateral flow tests are more than 80 per cent effective when it comes to detecting any level of Covid infection.

The study added that the figure rises to over 90 per cent when those who have the virus are at their most infectious.

It’s also been reported that lateral flows are able to detect the Omicron variant of Covid.

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The UK Health Security Agency said in December that the tests are as likely to detect the virus in those with the new variant as well as those with other strains such as Delta.

While the tests are said to be generally accurate, some experts have sounded a note of warning, pointing out that they work best if a person is asymptomatic – and that if a person has Covid symptoms they should always book a PCR test, even if they get a negative result on a lateral flow.

According to a statement from the Royal College of Pathologists lateral flow tests ‘are very different from PCR. They are not suitable for diagnosing individual patients who suspect they may be infected because they have symptoms.

‘People with symptoms need a PCR test. Lateral flow tests are intended for picking up additional infected cases who would otherwise be missed because they don’t have any symptoms.’

So, if you have any symptoms of Covid, it’s always best to get yourself a PCR test and self-isolate until you receive a negative result.

How long do lateral flow tests last?

Currently, any negative test results recorded in the NHS Covid pass app expire after 48 hours – while government guidance currently recommends taking a test ‘if you will be in a high-risk situation that day’.

However, experts are now suggesting that people should test immediately before going into any high-risk situation where they might infect others – as the Omicron variant is spreading so fast that a person can switch from being non-infectious to infectious ‘within hours’.

Regarding LFD. If you have symptoms and are LFD positive you have Covid. Don’t wait on PCR confirm to act. If you are LFD negative you still need to exercise caution. Protect yourself and others. And use LFD just before you go out/visit. Not within 24 hours. Just before.

Irene Petersen, a professor of epidemiology at University College London, told The Telegraph: ‘We’re seeing so many examples now where people have taken a test a day before and then when they take one the day after they are positive.

‘Omicron is very, very fast, so the test result expires very quickly. It is hours that we are talking about now.’

Alan McNally, professor of Microbial Evolutionary Genomics at the University of Birmingham, also backed this up, saying on Twitter: ‘If you have symptoms and are LFD (lateral flow) positive you have Covid. Don’t wait on PCR confirm to act. If you are LFD negative you still need to exercise caution. Protect yourself and others.

‘And use LFD just before you go out/visit. Not within 24 hours. Just before.’

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