What it means if Covid lateral flow test result shows a faint red line

By Claire Schofield
Tuesday, 28th December 2021, 5:00 pm
Updated Wednesday, 29th December 2021, 3:26 pm
A lateral flow test result is interpreted by the two letters on the device (Photo: Shutterstock)
A lateral flow test result is interpreted by the two letters on the device (Photo: Shutterstock)

The highly transmissible Omicron variant now accounts for 90% of all new Covid-19 cases in the UK and infections are still rising.

Despite the growing number of cases, further coronavirus restrictions will not be put in place in England before January, the Health Secretary has confirmed.

The decision is in contrast to the rest of the UK, with people in Wales, Scotland and Northern Ireland subject to stricter measures, including the closure of nightclubs and limits on social mixing.

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With New Year’s Eve celebrations looming, people are being urged to “remain cautious” and to take a lateral flow test before meeting up with others.

Daily testing is encouraged regardless of whether you are planning to go out celebrating or not, as a means of keeping track of cases and reducing the risk of further transmission.

But how do you interpret lateral flow test results? Here’s what you need to know.

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.

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.

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.

A faint line next to the T is likely to be a positive result

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.

"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 Covid-19 symptoms, you should take a lateral flow test and self-isolate at home until your results are confirmed.

If you do test positive, your self-isolation period includes the day your symptoms started (or the day you had the test, if you did not have symptoms) and the next 10 full days.

You can stop self-isolating after the 10 days if either:

  • you do not have any symptoms
  • you just have a cough or changes to your sense of smell or taste – these can last for weeks after the infection has gone

This rule applies to everyone in Scotland, Wales and Northern Ireland.

If you live in England, the self-isolation period for people infected with Covid-19 has been cut from 10 days down to just seven, depending on a test result.

A lateral flow test must be taken 24 hours apart on day six and seven of the isolation period, and if both tests come back negative, isolation can end on day seven.

This article originally appeared on our sister site, NationalWorld.