We’ve seen and been asked about a video in which Foreign Secretary Dominic Raab appears to say that Covid tests false positive rate is 93%.
In the video from September, Mr Raab is asked why there is not testing at airports currently. He responds:
“The challenge is that the false positive rate is very high so only 7% of tests will be successful in identifying those that actually have the virus. So the truth is that you can’t only rely on that.”
The false positive rate means the percentage of tests that wrongly tell people who do not have the virus that they are infected. Tests can also produce false negatives, where people who do have the virus are told they don’t.
It appears Mr Raab was referencing a PHE report, which stated that the success rate of identifying Covid cases when only testing travellers on arrival at airports is 7%. By testing on arrival and then testing again five days into self isolation, the success rate goes up to 88%.
Mr Raab appears to have mis-spoken, as the PHE report doesn’t talk about false positives at all.
It’s actually referring to false negative results: incoming travellers who are infected, but who might not be identified by a single test at an airport. This can be due to factors such as the virus’s incubation period, when an infected person might still test negative due to the low levels of virus in their body.
In addition, the PHE study is not based on real-life data. It is simply a modelling study intended to explore how different isolation and testing regimes might improve the chances of identifying infected travellers. Like all models, it simplifies things and makes some assumptions that may not reflect reality.
It’s not possible to establish a definitive false positive rate for Covid-19 testing, as this can vary according to circumstance, but as we have written elsewhere there is good evidence that false positives are extremely rare with the RT-PCR tests that are used for most testing in the UK.