Covid lateral flow tests: Doctor describes how to spot a false positive


As more people are turning to rapid antigen tests, also known as LFTs, confusion over interpreting the results correctly has arisen. An A&E doctor from London has taken to his Instagram to settle the debate on what the faint line next to the letter T on the small, white test actually means.

After carefully following every step on the test explanation leaflet, you are usually left with a pretty straightforward result.

The Covid test either reads positive, showing two lines next to the letters C and T, or negative, displaying only one line next to C.

However, if the line suggesting a positive result appears to be very faint, interpreting your result and taking further action can become confusing.

Dr Nathan Hudson-Peacock has explained how to read a rapid test with “a very faint line” on his Instagram account @expedition_doctor to settle the debate.

READ MORE: Cancer: The 60p ingredient consumed by millions in UK that’s ‘shown to cause cancer’ 

The doctor started the post by warning that anyone with symptoms of coronavirus should isolate until receiving a negative PCR result as PCR tests remain the most reliable testing option.

If you need reminding, the three main symptoms of coronavirus are new, continuous cough, fever, a loss or change to your sense of smell or taste.

Although these are the symptoms listed on the NHS website, the new variant called Omicron seems to be causing slightly different signs including:

  • Scratchy throat
  • Runny nose
  • Sneezing
  • Mild muscle aches
  • Extreme tiredness
  • Night sweats.



How to spot a false positive?

Dr Hudson-Peacock advised that if your rapid test shows a faint line next to the T, it’s important to consider the interpretation window.

He penned: “Essentially, if any line appears before the end of the interpretation window (check leaflet, usually this is 30 minutes), then 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.”

Does the faint line appearing after the interpretation window mean anything? 

The doctor said: “Note the following is my own views only. 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…to continue testing with LFTs as per NHS guidance.”

The current guidance advises taking a rapid test before mixing with others indoors. You also have to do a daily test for a seven-day period if you are a close contact of someone with Covid and you are asymptomatic and fully vaccinated.


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