Everyone in England is now able to take a free Covid-19 lateral flow test (LFT) twice a week, but experts have raised concerns about accuracy. We take a look at the issues.

What is a lateral flow test?

LFTs are a rapid way of testing for Covid-19. A swab is taken from the back of the nose or throat and it provides results a little like a pregnancy tests, with a red line appearing if coronavirus proteins are detected and a second line that indicates the test is working.

These tests differ from polymerase chain reaction (PCR) tests, which look for genetic material from the virus, called RNA (ribonucleic acid) and are more accurate. However, LFTs have the advantages of being cheap and offering results in about 30 minutes, and they can be carried out at home.

Where are LFTs used?

Until recently the tests were used for mass testing in particular settings, such as schools. However, earlier this month the government announced that LFTs would be made available for all adults to take a test twice a week. The idea is that the tests will help pick up cases of Covid that may otherwise go undetected, for example because of few or no symptoms, and help prevent the spread of the virus.

How accurate are LFTs?

The accuracy of LFTs appears to depend on the make of the test used, whether people have symptoms, and who is conducting the test.

The Department of Health and Social Care (DHSC) said the Innova lateral flow devices detect over 95% of individuals with high viral load, the most infectious cases, most likely to spread the virus further.

However, mass screening in Liverpool found that the devices detected only 67% of highly infectious people who had no symptoms, meaning one in three cases were missed. These tests were also “assisted” by experts, which have shown to be more accurate than those used by untrained members of the public.

A Cochrane review of 64 studies found that LFTs correctly identify on average 72% of people who are infected with the virus and have symptoms and 78% within the first week of becoming ill. But in people with no symptoms, that drops to 58%.

What are the concerns?

One concern is that those who receive a negative result from LFTs may still have the virus – experts have previously said this means the tests should be seen as offering a “red light” rather than a “green light”.

The regulator responsible for approving coronavirus tests for use in the UK, the Medicines and Healthcare products Regulatory Agency (MHRA), has expressed concern to government that people may be falsely reassured if they test negative on an LFT.

The MHRA has authorised LFTs to be used as a “red light” test – to find infectious people and order them to self-isolate – but not as a “green light” test to allow people greater freedoms if they test negative. The regulator is concerned that the government’s universal testing programme blurs the lines between the two.