An interesting comment in The Lancet Respiratory Medicine by Fearon and others published on June 14, 2021 discusses this issue.

Lateral flow device rapid tests for SARS-CoV-2 antigens are used for asymptomatic testing in various settings.  However, there is pressure within the government to rescind the asymptomatic testing due to concerns that despite a high specificity estimated to be 99.9%, the proportion of people testing positive who had COVID-19 was falling in line with reducing prevalence.  This leads to a greater proportion of individuals having to unnecessarily isolate because of a false positive test result.

The risk of people without COVID-19 self-isolating due to false positive test results is cost to the individual, their household and their workplace that needs very careful consideration and mitigation.

However, this cost needs to be considered in the context of the cost of failing to identify true positive results.  

In the United Kingdom, 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.  Discussion concerning lateral flow testing policy needs to incorporate the tradeoff between the negative effects of the false positives and the onward transmission prevented.  This tradeoff is particularly pertinent when considering the contribution of lateral flow tests to prevent the need for additional widespread restrictive measures.

Keeping COVID-19 prevalence low is of great public benefit.  During the pandemic, all people in the UK have been asked to take measures which might personally be challenging to mitigate risk to others even when they have no symptoms and low likelihood of transmitting the virus.  People in the United Kingdom generally wear a mask over their nose and mouth in enclosed spaces and self-isolate if they have been in contact with someone known to have COVID-19, even if just an estimated 10-15% of people who come into contact with someone with COVID-19 becomes infected during a period of high prevalence.  

These measures need to be considered analogous to responses to a false positive test, but the public recognises their value in the reduction of transmission.  Most people also recognise that reducing the risk of transmission to others is of benefit to themselves and the same applies to asymptomatic community testing.

No measures to control SARS-CoV-2 transmission are without cost or harm, and these costs and harms are not experienced equally across society.  If asymptomatic testing is to work and be equitable, it is imperative that more is done to ensure that isolation or quarantine is not an undue sacrifice but this proportionately affects people who cannot work from home and might lose their jobs, incomes, or ability to care for family members.  

The crucial part of this problem is distinguishing between false positive results and true positive results and their consequences as an end to end system.  Much of the harm of false positive results can be mitigated by taking a second test if the first is positive; if this is done via natural flow, it would add only 30 minutes and test patches or even with tests that detect different antigens would help address concerns that the chance of receiving a false positive result might be correlated across tests delivered together, especially if they are from the same batch.  Although a second test increases specificity of the testing procedure, it can only lower overall sensitivity as neither lateral flow test nor PCR testing is 100% sensitive.  The accompanying reduction in true positive results could also have an effect on transmission. 

If COVID-19 prevalence is low, and the proportion of false positive results is just too high for the mass asymptomatic population testing, when considered with the appropriate trade offs, the lateral flow testing might well be well suited to other applications, including; testing subpopulations with high prevalence, such as people who have been in contact with someone with COVID-19, testing in high transmission settings or where physical distancing is impossible; testing in areas where variants of concern have been detected.  

The role of lateral flow testing in society can, and should, be subjected to continuous study, review, and communication, with policy modifications made accordingly.  Furthermore, messaging about lateral flow test accuracy, interpretation and importance should be clear, and these should reach underserved groups, and should be based on the most up to date evidence. 

Asymptomatic testing interventions should not be dismissed on the basis of number of people isolating after false positive test results alone without assessing their worth in preventing further onward transmission and more widespread restrictive interventions. 

LGP, The London General Practice, the leading London Doctor’s clinic provides lateral flow testing and rapid PCR testing.  The lateral flow device that we use for rapid lateral antigen testing is approved by the government and if positives are found then we perform a PCR test which in most cases we can get back the same day.

Dr Paul Ettlinger
Founder, The London General Practice

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