Dr Clare Craig

Our testing regime is dangerously flawed – here’s how to fix it

False positives are skewing the data and harming our ability to counter the virus

(Photo by Christopher Furlong/Getty Images)

Matt Hancock has announced a £100 billion spending programme for mass population testing — his so-called ‘moonshot’ initiative, that would see 10 million tests delivered a day. Does this mean we need rocket boosters under the testing programme, fuelled by vast reserves of taxpayer cash? Thankfully, there could be a simpler and more pragmatic approach that not only saves money but would prevent a lot of harm.

There are currently two main problems. The first is a poor definition of what constitutes a case of coronavirus. The second is a testing strategy designed for the wrong point in the pandemic.

The government advisory group SAGE estimates that the standard tests used now have a false-positive rate of less than one per cent. But that’s quite a big range. Oxford’s Carl Heneghan has written before about the implications of a false positive rate of even 0.1 per cent. If the real number was closer 1 per cent – which SAGE seems to consider quite feasible – it would mean that if 100 people are tested using our current methods, around one person would be diagnosed as Covid positive even if they don’t have the disease.

If the UK was completing 200,000 tests a day (which is around the number the government says it’s currently carrying out), you would expect between 2,000 false positives a day. That accounts for the majority of our current positive results.

There have been 17.7 million tests carried out so far. A false positive rate of 1 per cent would mean that 177,000 of those positive results would be false positives. That’s nearly half of all the cases ever diagnosed in the UK.

This dilution of the seriousness of the disease is only part of the issue. Firstly, it has led to life, and indeed, world-changing political decisions based on false data.

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