Intended for healthcare professionals

Letters Covid-19 screening

Mass population screening for SARS-CoV-2 and false positives—why Liverpool shows we have a problem

BMJ 2020; 371 doi: (Published 31 December 2020) Cite this as: BMJ 2020;371:m4954
  1. David Cook, chief scientific officer
  1. Blueberry Therapeutics, Alderley Park, Macclesfield SK10 4TG, UK
  1. david.cook{at}

In The BMJ’s Editor’s Choice article published on 19 November,1 Abbasi details some of the problems associated with the UK’s proposed mass testing regime. Despite many concerns, not least the ethical issues raised,2 mass testing is now planned for the UK.

As Abbasi says, “Although the false positive rate [of the screen] is small, it is still a problem in a low prevalence setting.” The mass screening trial in Liverpool showed that only a small proportion of people with no covid-19 symptoms (0.7%) were positive for the presence of SARS-CoV-2.3 If representative of the UK as a whole, then we have precisely the conditions in which false positives become a problem—a low prevalence of infection in a largely negative population.

Clinically, we cannot tell the difference between an asymptomatic “carrier” and a false positive result from a healthy person, because, by definition, none of these people have symptoms. We are wholly reliant on the fidelity of our screening approach. But the effect of positive test results on individuals is substantial, requiring self-isolation and possibly leading to broader societal effects like closures of schools and businesses.

Screening tens of millions of people means that even a low false positive rate translates to a large number of uninfected people receiving a diagnosis of “asymptomatic covid-19.” A false positive rate of just 0.1% would produce an apparent infection rate of 100 “cases” per 100 000 healthy people screened.

If ongoing repeat screening becomes a requirement to live a normal life, as has been suggested,4 then the problem of false positives will multiply. Test an uninfected person enough times and eventually one of the results will be (false) positive.

Finally, we need to remember that if we do not account for false positives in screening, then rather than an ultimately self-limiting viral pandemic, we will have a never ending “pseudo pandemic” driven by testing artefacts.


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