Large-scale testing for SARS-CoV-2 by the reverse transcription polymerase chain reaction is a key part of the response to the COVID-19 pandemic, but little attention has been paid to the potential frequency and impacts of false positive results. In the absence of data on the clinical specificity of SARS-CoV-2 assays, we estimate a conservative false positive rate from external quality assessments of similar viral assays, and show that this rate may have large impacts on the reliability of positive test results. This has clinical and case management implications, affects an array of epidemiological statistics, and should inform the scale of testing and the allocation of testing resources. Measures to raise awareness of false positives, reduce their frequency, and mitigate their effects should be considered. outbreak of human coronavirus OC43 infection and serological crossreactivity with SARS coronavirus. Canadian Journal of Infectious Diseases and Medical Microbiology. 2006;17(6):330-6. 7 Koetz A, Nilsson P, Lindén MV, Van Der Hoek L, Ripa T. Detection of human coronavirus NL63, human metapneumovirus and respiratory syncytial virus in children with respiratory tract infections in south-west Sweden. Clinical microbiology and infection.