Can a lateral flow test give a false positive? Accuracy of rapid Covid tests - and can booster affect results?

Covid-19 cases in the UK reached record high levels over the festive period, as the Omicron variant rapidly spread.

The highly contagious strain, now dominant in the UK, has led to a surge in infections and has sparked a shortage of lateral flow tests as people scramble to get tested.

People have been relying on the rapid tests to check if they have the virus before meeting with friends and family over Christmas, and the government is advising that regular testing continues to help control transmission.

But how accurate are the tests at detecting Covid-19 cases? Here’s what you need to know about their effectiveness, whether you can test positive after your booster vaccine, and how to report the results.

How accurate are lateral flow tests?

Analysis from testing last year found that lateral flow tests have a specificity of at least 99.9% when used to test in the community.

Findings from NHS Test and Trace revealed that for every 1,000 lateral flow tests taken, there is less than one false positive result.

More recently, a study published in the Clinical Epidemiology journal in October found that lateral flow tests are more than 80 per cent effective at detecting positive coronavirus cases, while they are more than 90 per cent effective at detecting the virus when someone is at their most infectious.

In a document published in December, the UK Health Security Agency (UKHSA) said that lateral  flow tests are also capable of detecting the Omicron variant.

Can lateral flow tests give a false positive?

Regular lateral flow testing is an effective way of identifying coronavirus cases among people who are asymptomatic, but they are less reliable overall than a PCR test.

The tests have been associated with false results, with some people reporting tested negative but then later testing positive after taking a PCR.

However, the chances of getting a false positive result are very slim.

Dr Gary Bartlett told The Independent that getting a false positive from a lateral flow, meaning the test shows you have Covid-19 when you do not, is “extremely rare” and only occurs in a “fraction of a per cent of occasions”.

He said: “If you have Covid symptoms, lateral flows are less reliable as they can be associated with false negatives often leading the person to believe that they don’t have covid when in fact they do.

“False positive results will happen in a fraction of a per cent of occasions, but false negatives (a negative result when in fact you do have Covid-19) are more common.

“Therefore, if you have Covid-like symptoms but test negative on a lateral flow, it is a good idea to book in for a PCR test.”

Dr Mike Tildesley, from the University of Warwick and a member of the Scientific Pandemic Influenza Modelling group (Spi-M), agreed that lateral flows are very accurate at recording a positive result, while England’s chief medical officer Sir Chris Whitty said the tests are a “very good guide actually to whether someone is at that moment infectious”.

Can the booster jab make you test positive?

No. The Covid-19 cannot make you test positive for the virus.

None of the coronavirus vaccines contain live versions of the virus so they will not affect the test result.

While the booster jab will increase the number of antibodies to fight the virus in your body, none of the vaccines offer 100% protection, so it is still possible that you can contract the virus, even after your jab.

However, the vaccine will not influence the result of a Covid-19 test.

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