LFT Results: Your guide to the UK’s Covid-19 testing rules and how LFTs really work

With record numbers of people in England and Wales testing positive for Covid-19, many are keen to know what the lines mean when you have a positive lateral flow test and how they relate to your likelihood of infecting others

Health


April 6, 2022

A lateral flow test strip with Covid-19 test results

Richard Harding/Alamy

Record numbers of people in England and Wales are testing positive for Covid-19. Here’s what you need to know about how lateral flow testing works, why symptoms can persist even after a person no longer tests positive, and how long you can test positive after you’ve recovered from your symptoms.

What is the current advice in the UK if you have Covid-19?

Although people in England are no longer legally required to self-isolate if they have Covid-19 symptoms or have tested positive, the UK Government’s advice remains that they should try to do so for at least five days despite being contagious for up to 10 can last for days and should therefore avoid contact with people who are at higher risk for this period. “The focus of this new phase [of the pandemic] is about protecting those most at risk from the virus,” a spokesman for the UK Health Security Agency (UKHSA) said in a statement. In Scotland, Wales and Northern Ireland, people should only end isolation before 10 days if they have two negative results from a lateral flow test (LFT) over two days.

In England, people are no longer advised to take LFTs for screening if they go negative and the tests are no longer free for the general population, although they can be bought from pharmacies. “The fact that it’s no longer legally enforceable doesn’t mean that people shouldn’t still be careful,” says Christopher Chiu of Imperial College London.

How have the rules changed in England for children?

The UKHSA is no longer advising people under the age of 18 to get tested for Covid-19 unless it is on the advice of a doctor. For those who test positive, the recommended period of self-isolation has been shortened to three days after the day the test was taken. “There is some evidence that children have a shorter illness duration compared to adults,” UKHSA chief Jenny Harries said in a statement. “Ideally, children would return to school as soon as lateral flow goes negative,” says Iain Buchan of the University of Liverpool in the UK. “But at some point political decisions have to be made about costs and priorities. It is important to prioritize children’s education and social development.”

How do I count how long I’ve been infected?

The first day someone shows symptoms or tests positive is counted as day zero. Someone trying to self-isolate up to day five would actually stay home for six days.

Does the intensity of a line on an LFT say anything?

Lateral flow tests are not approved for this type of use, but people usually see the intensity of the line on their test changing from faint to dark and back to faint as their infection progresses. Some studies show that the intensity of the line correlates to the amount of virus particles in the person’s nasal fluid. “These tests are actually very quantifiable because of the blackening of the line,” says Michael Mina of eMed, a US testing company. But even if the line is faint, there still needs to be a replicating virus in a person’s body to produce enough protein to produce a positive result.

Why can I still have symptoms despite a negative test?

Some symptoms may persist after someone is no longer contagious. “In general, people have a cough for a long time,” says Al Edwards of the University of Reading in the UK. “There are two reasons for coughing. One is because you are infected and that causes damage and that makes you cough. The other reason you cough is because your airways are being damaged by the viral infection and are healing.”

Can you get false positive results from LFTs after a Covid-19 infection?

Schools in the UK are being told by health services that children can have a positive LFT up to three months after being infected. This is “theoretically possible, but it would be very unusual,” says Buchan, who conducted the first mass study of LFTs in Liverpool. LFTs test for viral protein produced by replicating viruses and are therefore less likely to give a false positive result than PCR tests. PCRs detect the virus’s genetic material, fragments of which can remain for several weeks after viable virus is no longer present.

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