SARS-CoV-2 antigen testing: weighing the false positives against the costs of failing to control transmission.

Lateral flow device (LFD) rapid tests for SARS-CoV-2 antigens are used for asymptomatic testing (including for people who are presymptomatic or paucisymptomatic) in various settings, including in the UK. As of April 9, 2021, LFD tests were made available for twice per week rapid testing to the general population in England. News articles reported pressures within the UK Government to rescind asymptomatic testing due to concerns that, despite high specificity (estimated to be 99·9%),1 the proportion of people testing positive who had COVID-19 (ie, the positive predictive value) was falling in line with the reducing prevalence, leading to greater proportions of individuals having to unnecessarily isolate because of a false-positive test result.2 Asking people to isolate on the basis of what might be a false-positive result is associated with a perceived unfairness and, in some cases, moral indignation. The risk of people without COVID-19 self-isolating due to false-positive test results is a cost to the individual, their household, and their workplace that needs consideration and mitigation. However, this cost should be considered in the context of the costs of failing to identify true-positive results. In the UK, the epidemic control strategies implemented during the past year, including lockdowns, have all, to varying extents, required people who do not have COVID-19 to isolate or quarantine and to greatly restrict their social contacts, while shutting down entire economic sectors. These restrictions have had massive implications for the incomes, education, and wellbeing of many people, including children and young people.3 Any discussions concerning LFD testing policy should incorporate the trade-off between the negative effects of false positives and the onwards transmission prevented. This trade-off is particularly pertinent when considering the contribution of LFD testing to preventing the need for additional widespread restrictive measures.