Low rate of false-positive rapid antigen test results in Canadian workplaces

A new study has examined the incidence of false-positive results in a large sample of rapid antigen tests used to screen asymptomatic workers throughout Canada. Conducted by researchers at the University of Toronto, the study was initiated because of concerns that rapid antigen tests for SARS-CoV-2 might result in false-positive test results and undermine pandemic management for COVID-19. The researchers obtained data from the Creative Destruction Lab Rapid Screening Consortium, which used rapid antigen tests for SARS-CoV-2 to help control transmission in workplaces throughout Canada. Asymptomatic employees were voluntarily screened twice weekly from Jan. 11 to Oct. 13, 2021. During this period, Canada experienced two significant delta variant-driven waves (March to June, and August to October). Read: Breakthrough COVID-19 infection rates higher among those with immune dysfunction Researchers recorded screening results, place of employment, the test used and (optionally) the lot number. For any positive test result, the patient was immediately referred for a confirmatory polymerase chain reaction (PCR) test to be completed within 24 hours. A false-positive result was defined as a positive screen on a rapid antigen test and a subsequent negative confirmatory PCR test. Any false-positive results were matched to lot number and test manufacturer. Results showed that 903,408 rapid antigen tests were performed over 537 workplaces. There were 1,322 positive results (0.15%); of these, 1,103 had PCR test information. About two-thirds of screens were trackable with a lot number. The study detected 462 false-positive results (0.05% of screens and 42% of positive test results with PCR information). Of these, 278 false-positive results (60%) occurred between Sept. 25 and Oct. 8, 2021in two workplaces (675 km apart) run by different companies. All of the false-positive test results from these two workplaces occurred with a single batch of Abbott’s Panbio COVID-19 Ag Rapid Test Device. The authors note that the overall rate of false-positive results among the total rapid antigen test screens for SARS-CoV-2 was very low, which is consistent with other, smaller studies. The cluster of false-positive results from one batch was likely the result of manufacturing issues rather than test implementation. The authors conclude that rapid antigen tests are unlikely to result in too many false-positives that could overwhelm PCR testing capacity in other settings. In addition, the authors point out that their results demonstrate the importance of having a comprehensive data system to quickly identify potential issues with rapid tests. With the ability to identify batch issues within 24 hours, workers could return to work, problematic test batches could be discarded, and the public health authorities and manufacturer could be informed. Reference 1. Gans JS, Goldfarb A, Agrawal AK, et al. False-positive results in rapid antigen tests for SARS-CoV-2. JAMA 2022; Published online January 7, 2022. (accessed January 11, 2022).

Low rate of false-positive rapid antigen test results in Canadian workplaces

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