Can lateral flow tests be false positive or negative?

Person holding a lateral flow test to symbolise whether you can get a false positive lateral flow test
Credit: Getty

Can lateral flow test be false positive or negative? It’s the question everyone is asking as a new wave of infections sweeps the UK. 

Lateral flow tests, otherwise known as lateral flow devices or LFDs, are one of the easiest ways to check whether you’ve got Covid-19. They take just 30 minutes to develop, a significantly shorter wait time than the 24-hour minimum turnaround of PCR tests. And while they’re predominantly supposed to be used for testing those without symptoms, many people are using them to test for Omicron symptoms as self isolation rules have changed for England.

So can lateral flow test be false positive? And when should you opt for a PCR test instead? 

Can lateral flow tests be false positive?

It’s unlikely that lateral flow tests (LFTs) will give out a false positive, research indicates. 

A Cochrane review from March 2021 assessed over 60 studies, all of which looked at the accuracy of various lateral flow tests. They found that the specificity of the tests was high, meaning they had a strong ability to accurately diagnose those who were not infected with the virus. 

A lateral flow test with two red lines indicating a positive result, which is likely not to be false

Two red lines on a lateral flow test indicate a positive result, Credit: Getty

This means that if you get two red lines on your lateral flow test, you’re almost certainly positive for Covid-19. In those who did not have Covid-19, the lateral flow tests correctly gave a negative result to 99.5% of people with symptoms and to 98.9% of people without symptoms. 

The UK Health Security Agency (UKHSA), an executive agency sponsored by the Department of Health and Social Care, also says that the chances of a false positive result are low – especially when there’s a high number of cases in the population. 

“The LFDs that we are using have a high specificity, that means the number of false positives are extremely low – less than 1 in 1000 tests conducted from our studies,” they say. “However, as the prevalence of Covid-19 reduces to low levels in the population, the chance of a false positive result increases – though still remains unlikely.” 

During these periods where the virus presence is low in the community, the UKHSA have urged people to confirm a positive lateral flow test with a PCR test. These are more accurate – by up t0 100% according to this Nature study. This is because the sample goes to a lab, rather than being tested at home. Here, it goes through a process that turns the virus’ RNA into DNA and makes millions of copies of that DNA. This in turn allows for the better identification of the virus.

There is naturally a very small chance that the lateral flow tests can be false positive – as with any test. But when the virus is highly present in the community, it’s highly likely that a positive lateral flow test is a real positive. 

Can lateral flow tests be false negative?

It’s significantly more likely that lateral flow tests will give out a false negative result, however. 

The same Cochrane review found that in people with confirmed Covid-19, lateral flow tests correctly gave a positive test to an average of 72% of people with symptoms. They correctly gave a positive test to 58% of people without symptoms as well. 

A lateral flow test with a single line, indicating a negative result

One single line on a lateral flow test indicates a negative result, Credit: Getty.

This means that 28% of people with symptoms who are given a negative result have an incorrect reading. While 42% of people without symptoms have an incorrect reading with a negative lateral flow test. 

Although, the research suggests that when you take the test has a big impact on your result. Tests were most accurate when they were used in the first week after symptoms first developed. This is likely because people have the highest viral load in their system in the first few days after infection. 

When should you have a PCR test?

You should get a PCR test if you have any symptoms of Covid-19. These include a high temperature, new and continuous cough, and a loss of taste or smell. 

However, the new variant is making up 90% of new coronavirus cases in England. And with the chance of us going back into lockdown before the end of the year if cases continue to rise, it’s worth checking out the symptoms of Omicron. These are much more similar to just a common cold.

It’s also because of the new variant that the government is upping the Covid booster jab programme, offering the vaccine to everyone over 18-years old in the UK. 

You should also get a PCR test if: 

  • You’ve done a lateral flow test and had a positive result.
  • Your lateral flow test had a void result (you couldn’t read it). 
  • You had a PCR test already but your test result came back void. 

If you don’t have any symptoms of Covid, you can also get a PCR test if: 

  • You’re a contact of someone who is positive and you have to self-isolate because you don’t meet the legal conditions. 
  • A local council or someone from Test and Trace has told you to get one. 
  • A GP or other health professional has told you to get a PCR test. 
  • You’re taking part in a government pilot project. 
  • You have to get a test to confirm a positive test result. 
  • You’ve have an unclear test result and need to get another test to confirm. 
  • You need a test for someone you live with who has symptoms. 
  • You’re part of the National Tactical Response Group.

Anyone who is a contact of someone infectious, and doesn’t need to self-isolate, must take a LFT for 7 days. If any of these tests are positive they should then self-isolate. 

Under the new rules, you’re only required to self-isolate if you have not had at least two doses of the Pfizer, Moderna or Oxford vaccine, or if you’re under 18-years and 6 months old. 

How long are you contagious with Covid-19?

If you’re confirmed to have Covid-19, you’re likely to be contagious for between four to eight days after infection. This is why the isolation period for those testing positive is 10 days in the UK, just to make sure. 

Video of the week:

The incubation period for Covid-19 lasts for between one and fourteen days. This is according to new data from the Office for National Statistics (ONS). While average time to develop symptoms is between five and six days. And 97.5% of those who do develop symptoms experience them within 12 days of infection. 

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