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

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.

Keywords: COVID-19; false positive results; high prevalence; low prevalence.

Fig 1.

Flow chart summarising the key findings of likely false positive and true positive results. *The range of Ct values in symptomatic patients 36–43, mean 39.5. One had a previous positive result and the result likely reflected residual genetic material. †One was most likely a true positive case. He was asymptomatic at the time of swabbing and tested positive. However, on admission he had CXR changes and lymphopenia. Although he initially tested negative on admission, this was likely a false negative. The other individual was asymptomatic at the time of swabbing and tested positive. Although he was negative on repeat swabs on the same day and two days later, he was positive three months earlier and was antibody positive. We therefore believe the positive result likely reflects the presence of residual RNA.