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Lateral flow tests: Isolation, results, faint lines and the rules you need to follow


New testing rules across the UK mean you don’t have to have a PCR test if you get a positive lateral flow test, as long as you do not have any Covid symptoms. It makes accurate LFTs more important than ever — you can read the new rules here.

Taking a lateral flow test sounds straightforward: a line next to the ‘C’ on the test simply means the test has worked, with the C standing for ‘control’, while a line next to the ‘T’ (test) indicates a positive Covid reading and you need to self-isolate.

But sometimes you can feel uncertain about the results of a lateral flow test or what to do depending on the result. At-home testing has reached record levels in recent weeks, with close to two million tests being taken on one day in January alone. So here we’ve clarified some of those issues that may cause some confusion.

Should I report a lateral flow result?

The public is instructed to report every result from a lateral flow test, no matter whether its positive or negative.

This helps scientists and governments get a better understanding of the spread of the virus across the country. If only positive results are recorded, the level of Covid-19 cases will look worse than it really is. Scientists also use the information to spot patterns and outbreaks more quickly and accurately.

The guidance on the Welsh Government website says that you need to report the result:

  • every time you use a rapid lateral flow test kit
  • as soon as possible after you get the result

If you are taking daily rapid lateral flow tests because you are a contact of someone who has Covid-19, you should report your result every day.

If you test positive when does self-isolation end?

So, firstly, if you test positive with a lateral flow test you need to self-isolate. But the self-isolation advice for people with Covid-19 has changed. It is now possible to end self-isolation after seven days, following two negative LFTs taken 24 hours apart. The first LFT should not be taken before the sixth day.

What are the other testing and isolation rules?

Here are the main principles of testing and isolation:

  • If you have Covid-19 symptoms you should stay at home and self-isolate immediately. You should arrange to have a PCR test as soon as possible. If this PCR test result is positive, you must continue to self-isolate.
  • If you do not have Covid-19 symptoms, but you have a positive PCR test result, you must stay at home and self-isolate.
  • If you are aged 18 years six months or over and you are not fully vaccinated, and you live in the same household as someone with Covid-19, you are legally required to stay at home and self-isolate.
  • If you are fully vaccinated or aged under 18 years and six months, and you live in the same household as someone with Covid-19, you are not legally required to self-isolate. However, you are strongly advised to take an LFT every day for seven days, and to self-isolate if any of these test results is positive.

What does a faint line on my lateral flow test mean?

After following the instructions on the leaflet included with testing packs, there’s often a clear result. A line next to the ‘C’ on the test simply means the test has worked, with the C standing for ‘control’, while a line next to the ‘T’ (test) indicates a positive Covid reading.

An A&E doctor has issued some guidance to help people understand what a faint line can mean. Go here to find out how to get a lateral flow test where you live. Dr Nathan Hudson-Peacock took to his Instagram page to explain. He said each testing kit will have an interpretation window, usually 30 minutes, but you can check the leaflet that comes with the kit to make sure. You should check your result within this time period to ensure it is accurate.

Dr Hudson-Peacock said: “You do a lateral flow test (LFT, aka rapid antigen test), and it shows a very faint line next to the T. What does this mean? Note: regardless of the LFT result, if you are symptomatic, you should isolate and book a PCR.”



The doctor’s own LFT test, showing a very faint line next to the T

“Essentially, if *any* line appears before the end of the interpretation window (check leaflet, often this is 30 minutes), then this is a *positive* test and you must isolate. The picture above is my positive LFT from this morning, and I have confirmed Covid.

“However, if a line appears *after* the interpretation window then this does NOT count as a positive test. NHS guidance is that you do not need to isolate and you do not need to book a PCR, unless of course you have symptoms.”



General picture of a Lateral Flow test
A lateral flow test showing a faint red line next to the T, indicating a positive result

“The next question is: does a faint line after the interpretation window mean anything? Note: the following is my own views only. If the faint line appears after the window, the most likely cause is either that there has been some contamination (e.g. food or drink, or some other weak contaminant), or there are just very low levels of the virus. If it is the latter, and obviously assuming you are asymptomatic at this point, then you are unlikely to be a transmission risk.

“Therefore, although NHS advice is not to isolate (bad for mental health and work etc) and not to book a PCR (makes it harder for people to get one), I suggest a sensible approach is to be extra careful with precautions (social distancing, hand washing and mask wearing), and to continue testing with LFTs as per NHS guidance, in particular doing another LFT before mixing with people indoors . Of course, if you develop symptoms, you must isolate.”

The doctor goes on to stress that NHS guidance is:

• Close contacts of Covid cases should do daily lateral flow tests (LFT) for seven days if both asymptomatic and fully vaccinated, or asymptomatic and 18 or under

• If not a close contact, then you should do a LFT before mixing with people indoors and before visiting someone who is at higher risk of getting seriously ill from Covid.

He adds: “Lastly, please remember, if you have symptoms, you should isolate and book a PCR, even with a negative LFT.”

Official NHS advice to staff says: “Leave your test for the full development time to get an accurate result. Do not read your results until 30 minutes. If the test device is left to develop longer you may receive a false positive result and you will need to repeat the test.”

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How do you report your result?

You cannot report a result after more than 24 hours. You can only report one result at a time. Go here to report your result. To use this service, you need:

  • the QR code or ID number printed on the test strip (the part of the kit that shows your result)
  • a mobile phone number so we can text you to confirm we’ve got your result

If you cannot use the online service, call 119 (free from mobiles and landlines). Lines are open every day 7am to 11pm.

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