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

Covid infections appear to be rising in parts of the UK, fuelled by the BA.4 and BA.5 Omicron variants

Covid infections have fallen in the UK for the first time in two months, marking the first drop in cases since late-May.

The decline marks the biggest sign yet that the current wave has reached its peak, although experts warn that prevalence of the virus still remains high.

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

Some 3.3 million people are estimated to have had the virus in the UK in the week to 20 July, a drop of 15% from 3.8 million in the previous week, figures from the Office for National Statistics (ONS) show.

Sarah Crofts, ONS head of analytical outputs for the Covid-19 infection survey, said: “Our most recent data suggest that we may now be over the peak of the latest wave of infections across the UK, although rates still remain among the highest seen during the course of the pandemic.

“We have seen welcome decreases among most parts of the UK and in all age groups.

“With summer holidays starting and more people travelling, we will continue to closely monitor the data.”

The current wave of infections has been driven by the BA.5 Omicron subvariant, which is now the dominant strain of Covid in the country.

However, high levels of coronavirus antibodies among the population – either from vaccination or previous infection – mean the number of people seriously ill or dying from the virus remains low.

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

People are being warned to assume that any cold-like symptoms is likely to be Covid, and are advised to get a test or stay away from other people 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.

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?

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.

"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

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.

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 that while the prevalence of the virus is “relatively high”, the risk of serious illness is lower than during previous waves.

He 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.”