What a faint line on a lateral flow test means and what you should do

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Over 28,100 Covid infections in the past 15 days in Malaga
Over 28,100 Covid infections in the past 15 days in Malaga. image: Wikimedia

There has been some uncertainty around what a faint line on a lateral flow test means and what to do about it.

There has been some uncertainty around what a faint line on a lateral flow test means and what to do about it, a line next to the ‘C’ on the test indicates it has worked, while a line next to the ‘T’ shows a positive result.

A&E doctor Dr Nathan Hudson-Peacock took to Instagram to explain the meaning, saying each kit will have an interpretations window that usually indicates results within 30 minutes.

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Dr Hudson-Peacock said: “You do a lateral flow test (LFT, aka rapid antigen test), and it shows a very faint line next to the T. What does this mean? Note: regardless of the LFT result, if you are symptomatic, you should isolate and book a PCR.”

“Essentially, if *any* line appears before the end of the interpretation window (check leaflet, often this is 30 minutes), then this is a *positive* test and you must isolate and book a PCR. The picture above is my positive LFT from this morning, and I have confirmed Covid.”

“However, if a line appears *after* the interpretation window then this does NOT count as a positive test. NHS guidance is that you do not need to isolate and you do not need to book a PCR, unless of course, you have symptoms.”


“The next question is: does a faint line after the interpretation window mean anything? Note: the following is my own views only. If the faint line appears after the window, the most likely cause is either that there has been some contamination (e.g. food or drink, or some other weak contaminant), or there are just very low levels of the virus.”

“If it is the latter, and obviously assuming you are asymptomatic at this point, then you are unlikely to be a transmission risk.”

Image – Instagram

“Therefore, although NHS advice is not to isolate (bad for mental health and work etc) and not to book a PCR (makes it harder for people to get one), I suggest a sensible approach is to be extra careful with precautions (social distancing, hand washing and mask wearing), and to continue testing with LFTs as per NHS guidance, in particular doing another LFT before mixing with people indoors. Of course, if you develop symptoms, you must isolate.”


The doctor goes on to say that NHS guidance is:

  • Close contacts of someone with Covidshould do daily lateral flow tests (LFT) for seven days if both asymptomatic and fully vaccinated, or asymptomatic and under 18
  • If not a close contact, then the person should do a LFT before meeting with people indoors and before mixing with someone who is at higher risk of getting seriously ill from the virus

He adds: “Lastly, please remember, if you have symptoms, you should isolate and book a PCR, even with a negative LFT.”

Official NHS advice to staff says: “Leave your test for the full development time to get an accurate result. Do not read your results until 30 minutes. If the test device is left to develop longer you may receive a false-positive result and you will need to repeat the test.”


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Laura is from a small seaside town in North Wales and has also lived in Liverpool and Manchester, where she studied English Literature and worked in social media and marketing. Laura moved to the city of Zaragoza last August to teach English, but after missing the coast she decided to move to beautiful Nerja to enjoy the sun and sea. Laura has a passion for animals, films, outdoor activities, writing and the environment.

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