Questions and Answers about Lateral Flow Tests for Covid-19

Lateral flow tests are now being used by many members of the public. In this article, I answer some of the common questions that people ask about these tests, which I was asked to comment on for an article for the Guardian Newspaper.

1. How do lateral flow tests work and how reliable are they?

Lateral flow test (LFTs) work by detecting the proteins on the surface of the SARS-CoV-2 virus that is the cause of Covid-19. They can be carried out at home and don’t require a laboratory to process the test, unlike PCR tests. They also give a result very quickly, typically within 30 minutes. They are less accurate than PCR tests, which is why these are recommended for people with symptoms.

Studies show widely varying estimates of the accuracy of LFTs, typically detecting between 40%-90% of people who have Covid-19 in different studies. We can’t just take an average of these studies to determine the accuracy of the tests as this will depend on a range of factors – such as the type of people in the study, what stage of illness they were at, what specific test was used, and how well the test was carried out.

We do know that carrying out the test correctly is very important. It’s essential therefore to read the instruction leaflet that comes with the test and to watch one of the NHS videos that explain how to carry out a lateral flow test to learn the correct technique.

 2. How long before meeting family/friends should you take one, and does taking multiple LFTs increase their accuracy?

The test should be done on the day of the meeting. One test is sufficient. Although you can carry out more than one test a day, there is a shortage of tests in many parts of England. If the tests are over-used, this will lead to further supply problems – including for key workers such as NHS staff who are required to test regularly for employment purposes.

 3. Does the thickness of the ‘test’ line on an LFT give any indication of how infectious you are? (if it’s a weak line, does that mean you are hardly shedding any virus? Does a thick one mean you are really infectious?)

The test is designed to tell you if you may be infected and is not designed to estimate how infectious you may be. You shouldn’t therefore use the thickness of the line to try to estimate your infectivity. At a time when the prevalence of Covid-19 in the UK is so high, a positive test means that you are very likely to be infected and you should isolate until you get the result of a PCR test; and then isolate further if this is positive.

 4. If you are still testing positive on an LFT after 10 days' isolation, are you still infectious and do you still need to isolate? (MIT medical journal says: "But what if someone still tests positive after 10 days? This is very common. People who have tested positive for COVID-19 are very likely to continue to test positive after 10 days. But they are not contagious.")

There is no need to keep on testing repeatedly if you have had a positive LFT result and this is then confirmed by a PCR test. You should complete your period of isolation and, if in England, carry out any required LFTs as advised by NHS Test and Trace. In England, people who are fully vaccinated can now carry out LFTs on day 6 and day 7 of their isolation period and come out of isolation if both these tests negative. Unvaccinated people will need to continue to isolate for 10 days. Test results can sometimes remain positive for a period after day 10 of isolation ends but this not necessarily mean you are still infectious. (UPDATE: From 17 January 2022, the isolation period in England was reduced to five days if the lateral flow tests are negative on day 5 and day 6).

 5. If you've tested positive, including for Omicron, does that mean you won't get it again?

Reinfections can occur after a previous Covid-19 infection. These are more common with Omicron which a recent study from Imperial College London showed was over 5-times more likely to cause a reinfection than the Delta variant. These reinfections will have been in people who have had an infection with a variant such as Delta, Alpha or the original version of SARS-CoV-2. Because Omicron infections have only occurred recently in the UK, we don’t yet know how prone people who have had an Omicron infection will be to a repeat infection.