Canadian study reveals rate of false positives from rapid antigen tests

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Toronto — As provinces rely more heavily on rapid antigen tests as part of their strategy to curb the spread of COVID-19, there have been concerns over the possibility of false positive results. Canadian researchers have produced new data shedding light on the likelihood of such events.

Researchers from the University of Toronto's Rotman School of Management published their peer-reviewed findings in the journal JAMA earlier this month. They looked at the results of more than 900,000 rapid antigen tests conducted over 537 workplaces in Canada between January and October 2021.

During this period, Canada had two significant waves of COVID-19 driven by the Delta variant. A total of 1,322 positive results were logged with rapid tests. Of these cases, 1,103 also had data from a PCR test to compare against.

In total, 462 rapid test results, or 0.05 per cent of the 900,000 results, resulted in false positives. This represents 42 per cent of the positive test results in the study.

Tests being administered too late in the infectious state or in an incorrect manner were some of the reasons that could explain these false positives, the researchers say.

Around 60 per cent of these false positives could also be traced back to issues stemming from one manufacturer. There were 278 false positive results from two workplaces that were all drawn from a single bad batch.

"The overall rate of false-positive results among the total rapid antigen test screens for SARS-CoV-2 was very low, consistent with other, smaller studies," the researchers wrote. "These results inform the discussion of whether rapid antigen tests will result in too many false-positives that could overwhelm PCR testing capacity in other settings.

The authors also say their findings illustrate the importance of "a comprehensive data system" to quickly identify bad batches of 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," they wrote.