It’s important to emphasise that there’s no strong evidence yet for any of the hypotheses advanced in this article. If the prevalence of Covid increases, the opposite would happen: we’ll see even more of these “surprising” testing sequences than currently. The percentage of people testing positive on LFTs who are true positives will drop and at the same time, the number of positive LFTs followed by negative PCRs will also plunge. In the calculations above, I’ve been conservative and assumed that 1% of people have Covid – the Office for National Statistics estimates that currently it’s actually nearly 1.5% of people.īut if this falls, everything changes. 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-19, 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. This could explain some of what’s been recently reported.īut this is only one half of the accuracy question. And because PCR tests are (almost) 100% specific, when they follow their LFT with one of these, they’ll then definitely get a negative result. In a population of 1 million people where 1% have Covid, 297 people will have LFTs tell them they have the virus when they don’t. Public Health England (now the UK Health Security Agency) has estimated that for every 10,000 LFTs taken by genuinely Covid-negative people, there will be fewer than three false positives (a specificity of 99.97%). This is largely because LFTs are very “specific” – they don’t give many false positives. As the diagram below shows, upwards of 96% (7,000/7,297) of people testing positive on LFTs at the moment will be true positives. Specificity and sensitivityĭespite their reputation for inaccuracy, if you test positive on an LFT, the overwhelming likelihood is that you have Covid. 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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