And to think, I was worried about a shortage of beds...(UPDATE)

[Celerity]

12. not true

False positives don’t really happen

The impact of false positive COVID-19 results in an area of low prevalence



https://www.rcpjournals.org/content/clinmedicine/21/1/e54

ABSTRACT

False negative results in COVID-19 testing are well recognised and frequently discussed. False positive results, while less common and less frequently discussed, still have several adverse implications, including potential exposure of a non-infected person to the virus in a cohorted area. Although false positive results are proportionally greater in low prevalence settings, the consequences are significant at all times and potentially of greater significance in high-prevalence settings. We evaluated COVID-19 results in one area during a period of low prevalence. The consequences of these results are discussed and implications for these results in both high and low prevalence settings are considered. We also provide recommendations to minimise the risk and impact of false-positive results.

Background

The UK's COVID-19 testing programme uses real-time reverse transcription polymerase chain reaction (RT-PCR) tests to detect viral RNA. Public Health England reports that RT-PCR assays show a specificity of over 95%, meaning that up to 5% of cases are false positives. The impact of false positive results includes risk of overestimating the COVID-19 incidence, the demand on track and trace, and the extent of asymptomatic infection. The adverse consequences of false positive results are proportionally greater in low prevalence settings.3 We therefore evaluated the clinical implications of false positive results in one area during a time of low prevalence.

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Conclusion

False positive results have the potential to cause harm in both high- and low-prevalence settings. Prevalence and the risk of harm needs to be considered when deciding on testing strategies. We believe that testing strategies need to be more agile and decisions on screening of various populations should be flexible and respond to the changing prevalence in the community or setting that is being investigated. Routine large-scale screening has the potential to cause the most harm in this respect and this risk needs to be balanced against the benefit that it will afford in any given setting. Large-volume screening at a time of low prevalence has the potential to do more harm than good and some large-scale screening strategies should be temporarily suspended when prevalence is very low. These strategies are likely to be of greater benefit in interrupting transmission during periods of high prevalence and we propose that they are re-instated when the prevalence in the community or particular settings warrant such an approach.

Careful interpretation of laboratory results is also required at all times, particularly in the context of screening asymptomatic individuals and cohorting positive patients. Low-level positive results in a single gene need to be interpreted with caution based on the clinical context. The Ct value can also provide useful information when assessing results and Clinicians need to become familiar with the interpretation of these results. Results should also be conveyed detailing the number of genes positive and the Ct value - not simply in a binary fashion (positive or negative).