What does a faint line on lateral flow test mean? Covid test results explained - and rules if it’s positive

The UK-wide total Covid infections has surpassed one million for the first time since late August

Covid infections are continuing to increase across the UK as health experts warn an autumn wave could be on the way.

Infection levels in England and Wales are still rising, but the trend in Scotland and Northern Ireland is unclear, according to the Office for National Statistics (ONS).

A lateral flow test result is interpreted by the two letters on the device (Photo: Shutterstock)

Some 1.1 million people in the UK have tested positive for coronavirus in the latest survey, covering the seven days to 17 September in England and the week to 20 September in the other three nations. This marks a 14% rise from 927,900 in the previous survey, which covered the week to 13/14 September.

It is the first time the UK-wide total has been above one million since late August, although it is still some way below the 3.8 million weekly infections in early July at the peak of the wave caused by the Omicron BA.4 and BA.5 subvariants of the virus. The figures come as the number of people in hospital with Covid is also continuing to rise.

Sarah Crofts, ONS deputy director for the Covid-19 infection survey, said it is “too early to identify whether this is the start of a new wave”.

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However, Professor Tim Spector, co-founder of the Zoe Health Study which is based on coronavirus symptoms reported by volunteers across the country, said on Thursday it is “clear we’re now seeing an autumn wave of Covid-19, combined with increases in hospital admissions”.

Symptoms of the Omicron variant are thought to manifest more quickly than previous strains, usually appearing within two days of being infected.

People have previously been warned to assume that any cold-like symptoms is likely to be Covid, and should stay away from others to minimise the risk of further spread.

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

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.

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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.

What does it mean if the red line is faint?

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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 that this is 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.

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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."

Should I self-isolate?

The legal requirement to self-isolate after a positive Covid test ended across the UK earlier this year, with the governments instead asking people to take “personal responsibility”.

In England, it is recommended that you follow NHS guidance if you feel unwell. The NHS says you should stay at home and avoid contact with other people if you:

  • have any symptoms of Covid and have a high temperature, or you do not feel well enough to go to work or do your normal activities
  • have tested positive for Covid

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If you have Covid you can pass on the virus to other people for up to 10 days from when your infection starts, although many people will no longer be infectious to others after five days. Those who test positive are urged to avoid meeting people at higher risk from Covid for 10 days.

If you live in Scotland, the Scottish Government recommends following the advice on NHS Inform.

If you test positive, you should stay at home and avoid contact with other people for five days after the day you took your test, or from the day your symptoms started (whichever was earlier). If you have not tested positive, you should try to stay home until you feel better.

In Wales, you should self-isolate and order a test if you display Covid symptoms and continue to self-isolate until you get your result.

If the test is negative you can leave isolation immediately, but if it is positive you should isolate for five full days and then take another test, plus another the following day. If both are negative you can leave isolation. If either one is positive, you should continue isolating until you get two negative results in a row, or until day 10, whichever is sooner.

In Northern Ireland, people are advised to isolate immediately if they have Covid symptoms or have tested positive.

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If you display Covid symptoms you should self-isolate, order a test and remain in isolation until you get your result. If the test is negative you can leave isolation immediately.

If it is positive, you should stay at home and avoid contact with people for five days after the day of the test, or from the day symptoms started - whichever was earlier.

As children tend to be less infectious than adults, this period is reduced to three days for children and young people under the age of 18.

Health Minister Robin Swann said: “This updated advice seeks to strike the right balance at this stage of the pandemic between reducing transmission, protecting the vulnerable and mitigating the disruption caused by longer periods of isolation.

“As we move forward together, and continue learning to live life Covid aware, I would urge people to use personal judgment, to act responsibly and to take sensible actions to help stop the spread of Covid-19 and other respiratory infections. This in turn will help to protect those who are most vulnerable.”