It’s possible that part of the increase in reports of this perceived-to-be-unusual event are actually the result of this positive feedback loop.Ī second thing to note is that how well (or poorly) LFTs and PCRs perform overall and relative to each other depends on how prevalent COVID is in the community. Reports of positive LFTs followed by negative PCR tests have been flooding Twitter in recent days and making national news, which in turn has caused more people to come forward. When something that we believe to be very unlikely happens to us, then we tend to try to make sense of our experience by sharing it with others. This may explain why reports of this positive-negative testing sequence seem to be swelling in number. This is perhaps higher than we might expect, given the bad rep of the LFT and the “gold standard” status of PCR tests. The first is that the rate of infected people testing positive on an LFT and then negative on a PCR test is around 3.5% (350 out of 10,000). There are a few further things to note here. And if you’re symptomatic, the chance of being infected if receiving a positive LFT followed by a negative PCR is even higher. And even if your positive result is followed by a negative PCR test, it’s currently more likely that you have COVID than don’t (350 vs 297). It’s important to remember that because of their high specificity, you can be pretty confident that a positive LFT result is genuine – in our model above, for every 7,000 that are right, only 297 are wrong. Of these, 5% – so 350 people – will then get an incorrect false negative from their “confirmatory” PCR test. As the diagram above shows, with the current prevalence of COVID, 7,000 of our 1 million people will correctly be flagged as having COVID by an LFT. PCR tests are much better, with a false negative rate of only 5%.īut this 5% false negative rate can also lead to a positive-then-negative testing sequence. Estimates vary, but perhaps around 30% of the time when someone has COVID, an LFT won’t pick this up. LFTs have a gained their bad reputation because of their low “sensitivity” – meaning they have a high rate of false negatives. Kit Yatesīut this is only one half of the accuracy question. Possible testing outcomes for 1,000,000 people in the population using representative values for test specificity and sensitivity and a 1% community prevalence value. It’s all down to how often these tests give false positives and negatives. Alternatively, vaccination may have changed where exactly in the body virus grows best, meaning different swabbing techniques used for different tests types are capturing more or less of the virus.īut there’s also a potential mathematical explanation, given neither test is 100% reliable. These positive-then-negative sequences of tests might be something to do with the way children are being tested. The increase in these events also coincides roughly with the return of schools and big rises in the number of cases in children. These spoofers would subsequently test negative on a follow-up PCR test. ![]() There have also been well-publicised stories of children faking a positive LFT result using the acidic properties of soft drinks. Others have hypothesised a new variant could be circulating that isn’t detected by the standard PCR test. Some have suggested a faulty batch of LFTs could be causing people to test positive when they don’t have COVID. A number of explanations have been put forward.
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