Does the thickness of the line on an LFT tell you how infectious you are?ĭouble lines appearing on an LFT – however faint – mean you have detectable virus in your nose and are therefore infectious. Petersen emphasises that “no test offers 100% accuracy, so please consider the risk of getting severe illness among your friends and family”. However, it’s worth bearing in mind that there is a test shortage at the moment, and they are urgently required for key workers, so use them strategically. ![]() If you took one early in the morning and are going out in the evening, you might take another, provided there has been a gap of six to 12 hours. “Omicron replicates very fast, so you may become very infectious within hours,” says Petersen. You could become infectious after you take the test, which is why scientists recommend taking it as close to your social plans as possible, ideally within an hour. How long before meeting family and friends should you take one? However, the gold standard remains the polymerase chain reaction (PCR) test, which detects viral load and infectiousness at lower levels, including before and after the time range in which they are picked up by LFTs. The main benefit of LFTs is they can be carried out quickly and easily at home, with a result in under 30 minutes, and that they don’t need to be sent to a lab. After carrying out tests, the UK Health Security Agency said LFTs have a “comparable sensitivity” for Omicron as for other Covid variants such as Delta. A recent paper demonstrated that LFTs, when used correctly, are likely to have a sensitivity above 80% and in many cases above 90%. The reliability of LFTs depends in part on whether you follow the instructions carefully, which is why studies give varying estimates of their efficacy, from 40% to 90%. “Please continue to test on a daily basis,” urges Irene Petersen, professor of epidemiology and health informatics at University College London. If you have come into contact with an infected person, it could take a few days for a positive result to show up. LFTs identify specific viral proteins that are present inside your nose when coronavirus is infecting you. “However, we would urge parents to ensure that tests are not being misused, and we would suggest to pupils who are interested in chemical reactions that the best place to learn about them is in chemistry lessons in school.How do LFTs work and how reliable are they, including for Omicron? We are sure this involves a very small minority of pupils, and that for the most part the tests are used correctly. Geoff Barton, the general secretary of the Association of School and College Leaders, said: “Misusing lateral flow tests in any way is massively unhelpful and we would strongly discourage this behaviour. ![]() While the faking of positive lateral flow tests may cause concern, not least because everyone in a household must self-isolate immediately should a test show a positive result, the disruption may be short-lived: anyone with a positive test must subsequently take a more accurate PCR test to confirm an infection. After this process the “positive” line in a faked test disappears, revealing the negative result. Writing in the Conversation, Mark Lorch, a professor of science communication and chemistry at the University of Hull, said it was possible to spot a “faked” positive test by washing it with a buffer solution that restores the correct pH to the device. ![]() ![]() There are less harmful ways to fake a day off school,” he said. “False positives affect not just that child but their family and their bubble at school, so pretty selfish thing to do. Jon Deeks, a professor of biostatistics at the University of Birmingham, criticised the practice. According to the i newspaper, there have been more than 6.5m views of videos uploaded to the social media platform under the search term #fakecovidtest. The “trick” has also been featured on TikTok, with videos of people trying out different fluids.
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