Rapid COVID-19 tests show low rate of false positives


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The false positive rate of rapid antigen tests for COVID-19 in the workplace was 0.05%, based on data from around 900,000 tests.

“Concerns have been raised about whether rapid antigen testing for SARS-CoV-2 may lead to false positive test results and undermine management of the COVID-19 pandemic,” but data from large samples do default, writes Joshua S. Gans, PhD, of the University of Toronto, Canada, and colleagues.

In a study published in the Journal of the American Medical Association, the researchers analyzed the results of 903,408 rapid antigen tests performed for 537 workplaces between January 2021 and October 2021. The tests were used to serially screen asymptomatic workers at various workplaces in Canada. Participation was voluntary and participants were screened twice weekly.

Recorded drug test results included an anonymized case ID, workplace, test, and sometimes batch number, which was available for about two-thirds of tests.

In the event of a positive test, the person was referred for a polymerase chain reaction (PCR) test to be carried out within 24 hours.

“A false positive result was defined as a positive screen on a rapid antigen test and a subsequent negative confirmatory PCR,” and false positive results were matched to test manufacturer and lot number, where applicable.

Overall, the researchers found 1322 positive results (0.15%). Of these, 1,103 had PCR information and 42% of positive tests were confirmed by PCR. Lot numbers were available for about two-thirds of the remains.

Notably, 60% of false positives occurred at two workplaces run by different companies between September 25 and October 8. The work sites were 675 km (421 miles) apart. All false positive tests from these locations were drawn from the same test lot: Abbott’s Panbio COVID-19 Ag rapid test device.

“The cluster of false-positive results from one batch were likely the result of manufacturing rather than implementation issues,” the researchers note.

The researchers also highlighted the importance of thorough data collection to quickly identify test quality issues and other issues. “With the ability to identify batch issues within 24 hours, workers could return to work, problematic test batches could be rejected, and public health authorities and the manufacturer could be notified,” they point out. Other causes of false positives include testing done too early or too late in the infectious stage and quality issues in self-administered testing, they added.

Study results were limited by several factors, including use of a convenience sample of workplaces, incomplete reporting of PCR confirmation results, and lack of lot number identification for approximately one-third tests, write the researchers. They also noted that the results reflected the Canadian epidemiology of COVID-19 during the study period and may not be generalizable to other countries and time periods.

However, the overall low rate of false-positive results for screening tests reflects data from earlier smaller studies, and the results of the current study may inform discussions about the risk that too many rapid tests will falsely positives overwhelms PCR testing capacity, the researchers concluded. .

Expert shares tips for rapid testing

“I’m not surprised by the results of the current study,” said Robert Glatter, MD, of Lenox Hill Hospital, New York, and editor-in-chief of Medscape Emergency Medicine. “The study confirms what we already know: the risk of a false positive is extremely low when performing rapid tests,” he said. “If anything, you would expect a false negative if you sample too soon after exposure or before symptoms develop. In a nutshell, if you test positive on a rapid test, you may have a degree confidence in the outcome,” Glatter said. .

“Rapid tests are a reliable indicator of infectiousness, which typically peaks about 1 day before symptom onset and then declines within a week of symptom onset,” Glatter explained. It is important to remember that rapid tests detect viral proteins that replicate in the nasal passages. These tests are specific enough to measure its degree of infectiousness and its ability to transmit the virus to others, he added.

“We also know that PCR tests, very sensitive tests capable of finding small pieces of viral genetic material, can detect a “dead” virus, that is, a virus that is not able to replicate. , but which can produce a positive result for several weeks after recovery, although the person being tested is not contagious,” Glatter said.

The Centers for Disease Control and Prevention now recognizes that the infectiousness period of COVID-19 extends beyond 5 days, “but they trust people wearing masks to reduce and prevent transmission from days 5 to 10 “said Glatter.

The clinical takeaways, according to Glatter, are that if a person has symptoms, takes a rapid test, and is positive, they can trust the result. “However, the presence of symptoms does not necessarily mean that you are infectious, since you can be infectious without having symptoms. In fact, 30 to 40 percent of cases are asymptomatic,” Glatter noted. “The relationship between having symptoms and being infectious is not always clear. What we know based on Omicron is that the onset of symptoms may actually occur before infectiousness develops. “, he added.

For people with negative rapid test results who remain asymptomatic, but have had recent exposure (within the last 5-7 days), the rapid test “may be falsely negative, and you should retest in 2 days , while still in quarantine,” Glatter advised. “If you are on day 6-7 of isolation, you are asymptomatic, but you see a faint red line in your rapid test, you are still infectious and therefore can transmit the virus,” he pointed out. . In that case, “test again in 1-2 days, stay isolated and continue to wear a mask,” he said.

Glatter added that the intensity of the red line on rapid tests correlates with infectivity, and therefore with the degree of viral replication, which indicates an individual’s ability to transmit the virus to others.

“Manufacturing or batch issues are usually the explanation for false positive tests, as opposed to implementation issues,” Glatter said. “In addition to batch issues, false positives can occur due to when the test is administered, either too early or too late in the infection stage, or the result of mechanics, or the way the test has been self-administered,” he said. . “In general, employers can be confident in the accuracy of rapid tests when used as a measure for safe return to the workplace after a positive test or exposure,” he noted.

Glatter also advised that “HCWs who remain asymptomatic and have a negative rapid test – after a recent prior positive test or exposure within the last 7-10 days (without the need for anti-fever medication within past 24 hours) – can safely return to work in the healthcare setting.

People “who continue to have minimal symptoms but feel well (without fever after a positive test or exposure in the last 7-10 days) but who continue to have negative rapid tests in isolation, are considered to be at risk. low risk and can return to work, provided they use recommended PPE,” he added.

The study received no external funding. Researchers and Glatter did not disclose any relevant financial relationship .

JAMA. Published online January 7, 2022. Research Letter

Heidi Splete is a freelance medical journalist with 20 years of experience.

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