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

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Covid and other winter viruses such as flu are now circulating more widely

Covid infections are rising again in most parts of the UK, with cases in England increasing for the third week in a row, new figures show.

Data from the Office for National Statistics (ONS) shows that infections in England are estimated to have climbed above one million for the first time since the end of October, while both Wales and Scotland have also seen an increase. The ONS said the trend in Northern Ireland is uncertain and there is a mixed picture among different regions and age groups.

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Health experts believe that people mixing indoors is likely to have driven the recent jump in infections, which comes as other winter viruses like flu are circulating more widely.

An estimated 1.3 million people across the UK tested positive for Covid in the week to 5 December, according to the ONS, up 16% from 1.1 million in the previous week. During the main waves of the virus earlier this year, the total peaked at nearly four million in July and just under five million in March.

The latest figures suggest the UK is facing its third winter in a row with Covid on the rise, but unlike the previous two years, when coronavirus was the main driver of sickness and hospitalisations, other viruses are becoming more prevalent.

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

Flu is now “circulating widely” across the country, with a sharp increase in hospital cases last week, particularly among the over-85s and children under five, according to the UK Health Security Agency (UKHSA).

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In England, the rate of hospital admissions for flu has overtaken those for people with Covid for the first time since the pandemic began, and is now running at a higher level than in any week since the 2017/18 season.

Dr Jamie Lopez Bernal, UKHSA consultant epidemiologist for immunisation, said: “We’re seeing rises in flu, Covid and other winter viruses as people mix more indoors. Covid hospitalisations are highest in the oldest age groups, so it is particularly important that everyone who is eligible continues to come forward to accept their booster jab.

“While Covid-19 and flu can be mild infections for many, we must not forget that they can cause severe illness or even death for those most vulnerable in our communities.”

Covid can spread more easily than flu and tends to cause more serious illness, but flu shares many of the same symptoms, making it difficult to tell the two apart. If you feel unwell and suspect you might have Covid, here’s how to interpret lateral flow test results and what to do if it comes back positive.

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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.

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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 that this is a positive resultA faint line next to the T is likely that this is a positive result
A faint line next to the T is likely that this is a positive result | Other 3rd Party

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."

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"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."

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”.

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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

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.

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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.

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.

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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.”

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