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 are started to increase across the UK once again, driven by the newer BA.4 and BA.5 Omicron variants.

The variants were first detected in South Africa at the beginning of the year and now appear to be spreading more quickly than other strains.

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The highly infectious BA.2 variant still remains the dominant strain in the UK but these newer variants look set to overtake, with BA.5 now already dominant in Portugal.

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

New ONS data shows all four UK nations have seen a slight rise in the prevalence of Covid in recent weeks, although the trend is described as “uncertain” in Scotland and Wales.

Some 797,500 people in private households in England were estimated to have tested positive for Covid in the week to 2 June - the equivalent of around one in 70, the Office for National Statistics (ONS) said. This is up week on week from 784,100, which was also about one in 70.

Infection levels are estimated to have gone up in London, north-west England and south-east England, while eastern England has also shown early signs of an increase.

Wales has seen Covid cases increase very slightly to 40,500 people, up from 39,600. Both estimates are equivalent to around one in 75 people and the ONS describes the trend here as “uncertain”.

Meanwhile, separate figures show the recent fall in the number of people in hospital with the virus may have come to a halt.

A total of 4,602 patients in England had Covid on 13 June, up 12% on the previous week, while in Wales the figure appears to have levelled off at around 250 to 260.

Patient numbers in both nations had previously been on a steady downwards trend since early April, following the peak of the Omicron BA.2 wave, but they remain well below the levels reached in all previous outbreaks.

Symptoms of the Omicron variant are thought to manifest more quickly than previous strains, usually appearing within two days of being infected, with common signs including a runny nose, fatigue, headache, dizziness, fever and chills.

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 you test positive you should isolate for five full days, starting from the day after you took the test, and then take another.

If that test is negative, take another test the following day and if that is also negative you can leave isolation.

If either test is positive, continue isolating until you get two negative tests in a row, or until day 10, whichever is sooner.