According to New Scientist, newscientist.com/article/23... they are just as reliable at detecting the Omicron variant of the SARS-COV2 virus as they are at detecting Delta and earlier variants - which is to say, not reliable enough to tell someone they are Covid-free!
Some salient points from the above article:
"Various studies have put the sensitivity of LFTs – in other words, their ability to detect the virus if it is there – at about 40 to 60 per cent. That sounds unhelpfully low, but it is an unfair measure as it compares LFTs to PCR tests, which are arguably too sensitive".
"People should use a positive result as a “red light” for stopping a social activity, but they shouldn’t use a negative result as a “green light” to go ahead with an activity that is potentially risky, as they might have some mild covid-19 symptoms... If your ‘pre-test’ decision is not to do the activity, then I would not generally recommend letting a negative result change your mind".
"Positive lateral flow test results can show an extremely faint band, which some people might incorrectly read as a negative test result".
"The crucial thing to remember is that if you have covid-19 symptoms, a negative LFT doesn’t give you the all-clear: you still need to use a PCR test".
"It is always possible to test negative and then go to positive.. Because of this, it is better to test just before mixing with other people, rather than relying on a negative test from the day before".
That last point is EXTREMELY relevant to testing your house guests etc over Christmas. I think it is better to get them to test on the day, and the day before, and the day before that.
Even if a one-off LFT has a sensitivity as high as 80 percent, and your guests have been less than careful, you are running a high risk that one of them brings Covid with them.
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bennevisplace
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bennevisplace, ummm, as I read and reread some parts, my head began to "spin". I had to remind myself, the "experts" are still working to get conclusive results. I am glad they are trying and not giving up. 🙂Sandra
Hi Sandra. I posted this article because in the UK so much political capital has been invested in lateral flow tests (along with vaccines of course). They have been key to getting people back to work and the economy moving.
A quick and accurate test, telling you if you are infectious, is the holy grail. It's not that simple. The reason conclusive evidence of their utility and reliability is so elusive is that much depends on how people use LFTs in real life, something no lab test or algorithm is going to pin down. This article highlights what official advice has brushed under the carpet, misleading people into taking risks they thought a negative test result had eliminated.
(Includes 6 minute video explaining how to do a rapid antigen test and how they work)
Rapid antigen tests instead detect viral proteins. The proteins bind in the solution to antibodies that become fluorescent to indicate the presence of the proteins.
Rapid antigen tests are:
- quicker than PCR tests (15-20 minutes versus hours to days to get a result)
- can be done in the home compared to having to line up and wait for a swab, which then has to be analysed in a laboratory.
- But they're less sensitive than a PCR test because there is no amplification process
The US Federal government has announced a rollout of 500 million free rapid antigen tests to US homes, commencing in January 2022, to help combat the Omicron spread.
Challenge Trial Shows People With COVID Shed Virus After Just 2 Days
Thanks. The Medpage article understandably majors on the USA.
I wanted to highlight the situation here in the UK where the Innova LFT is in general use. This test too seems to perform as well on Omicron as on other variants. But...
Innova claims a sensitivity of 97 % based on a study at the government research facility at Porton Down. This BMJ article, reviewing real-world performance, was less bullish and counsels caution: Clear and accurate communication with the public about how to interpret test results is important, given the chance of missing some cases, even at high viral loads bmj.com/content/374/bmj.n1637 The more recent article in New Scientist reinforces the point.
Sadly, in the UK official communication about the use of LFTs has been absent or distorted.
Comments appearing in the news feeds warn of not eating or drinking for around 30 minutes before a LFT test. Also what if you regularly have very dry nasal passages? They say to blow your nose but often this doesn't help me.I do think the throat swabbing was better in addition to the nose.
If you can identify the LFT you are using, from the information on the box of test kits, Annex 1 in this link ec.europa.eu/health/sites/d... should give you the test sensitivity (how good the test is at detecting an infection and avoiding a false negative result) and test specificity (how good the test is at detecting the absence of infection and avoiding a false positive result) in clinical use, as measured by independent laboratories.
For example, NHS Test and Trace supplied me with a box of 7 "SARS-COV-2 Antigen Rapid Test (Self-Testing)" branded Flowflex Ref L031-118Y5 and made by Acon Biotech (Hangzhou) Co. Ltd. Covid-19 Self-Test. A rapid test for the detection of SARS-COV-2 nucleocapsid antigens in anterior nasal swab specimens. This test had independently measured clinical sensitivities:
Days < 7: 92.2%; Ct < 33: 98.3%; Ct < 25: 100% where sensitivity increases with viral load (Ct decreases with viral load).
For most tests, the measured specificity is usually in the range 99-100 %, meaning that if you get a positive result you can be very confident you have the virus.
The measured sensitivity of tests varies from one to another. Some (like mine) take a nasal swab, others both nasal and throat swabs. Measured sensitivities are usually in the 90-99 % range. In practice they can be much lower than that: if you are pre-symptomatic with a small viral load; if you haven't swabbed your nasal cavity thoroughly (it can be uncomfortable to go in deep!); or if you have misread the readout (the positive line is sometimes faint). Which is why in the New Scientist article newscientist.com/article/23... epidemiology researcher Prof Irene Peters says "most studies of LFTs suggest they are somewhat more accurate at detecting positive cases in people who have symptoms than in those who are symptom free" and "if you have covid-19 symptoms, a negative LFT doesn’t give you the all-clear: you still need to use a PCR test. But LFTs for people with symptoms can be helpful while they are waiting for a PCR result, especially if PCR tests are hard to access, as they are at the moment".
While the UK study cited in the above did not find that LFTs performed significantly worse in the presence of the Omicron variant, a study in the USA did suggest that LFTs were often producing false negative results dailymail.co.uk/news/articl...
In summary, a Lateral Flow Test does a good job of avoiding false positive results, giving confidence that a positive result is accurate.
LFTs are certainly less good at avoiding a false negative result, and unreliable in the absence of symptoms. So if your result is negative, repeat the test the next day and so on, and if you have symptoms take a PCR test.
Do not be misled by generalised claims like "LFTs are accurate", and do not depend on a visitor having taken a one-off LFT before they come and have dinner with you.
This article explains with examples, three key concepts to understanding how accurate RATs/LFTs are: sensitivity, specificity and pre-test probability. Also included is a video by an ear, nose and throat surgeon, illustrating how to best perform sampling.
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