Covid-19: What does a faint line on a lateral flow test mean and do I need to isolate if I test positive?

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COVID cases are increasing in the UK, with infections rising following the easing of restrictions last month.

New figures from the Office for National Statistics (ONS) show that infections have risen in all four nations of the UK, with levels in Scotland at a record high.

It is the first time since the end of January that all parts of the country have seen a simultaneous week-on-week increase in infections, and is the clearest indication yet that the virus is again becoming more prevalent.

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If you suspect you might have Covid, here’s how to interpret the test result and what to do if it comes back positive.

Performing a lateral flow test .pic Richard PonterPerforming a lateral flow test .pic Richard Ponter
Performing a lateral flow test .pic Richard Ponter

What does the ‘C’ and ‘T’ mean on a lateral flow test?

A lateral flow test result is interpreted by the two letters on the device where the swap sample is dripped on to.

Above the ‘S’ where the sample goes, there is a section above with the letters ‘C’ and ‘T’ to the right.

If a red line appears next to the C, this means the test is negative.

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A test which returns no red lines at all, or just one line next to the T, means it is void and a new one will need to be taken on a fresh kit.

If your test returns two red lines - one next to the C and one next to the T - this means that it is a positive result and you had Covid-19 when it was taken.

You should take a PCR test to confirm this result and self-isolate at home until you know for sure.

What does it mean if the red line is faint?

If your lateral flow test returns a red line next to the C and a faint line next to the T, it is likely that this is a positive result - even if the T line is barely visible.

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London-based A&E doctor Nathan Hudson-Peacock warned that any line which appears within the interpretation window - which is usually 30 minutes, but the leaflet inside the test box should confirm how long - is classed as a positive test.

The doctor shared a picture of a test with a very faint positive line on his Instagram page earlier this month and explained in a post: ‘Essentially, if any line appears before the end of the interpretation window this is a positive test and you must isolate and book a PCR.

‘However, if a line appears after the interpretation window then this does not count as a positive test. You do not need to isolate and you do not need to book a PCR."

‘If the faintly positive line appears after the time window, the most likely cause is either that there has been some contamination (e.g. food or drink, or some other very weak contaminant that is causing a false positive), or there are just incredibly low levels of the virus.

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‘If it is the latter, and obviously assuming you are asymptomatic at this point, then you are very unlikely to be a transmission risk anyway and so it is of little significance.

‘Therefore, the most sensible next step, in my opinion, is not to isolate unnecessarily and not to book a PCR (makes it harder for people who genuinely need them to get one), but to be extra careful with precautions (social distancing, hand washing and mask wearing), and to continue testing with [lateral flow tests] as per NHS guidance.’

When should I self-isolate?

If you feel unwell or have any coronavirus, you should take a lateral flow test to check if you have the virus.

If you do test positive, the self-isolation period differs depending on where you live in the UK.

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If you live in England and Northern Ireland, you are not legally required to self-isolate if you test positive for Covid.

However, it is still strongly advised that you stay at home for at least five days and do not go into work, although this will not be enforced by law.

The requirement to take daily tests if you have been in close contact with someone with coronavirus in England has also been scrapped, regardless of your vaccination status.

A message from the Editor, Mark Waldron

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