COVID-19: Manchester antigen test results linked to slew of false positives

A cluster of COVID-19 antigen tests in Manchester has turned out a high number of false positives.

The Vermont Department of Health first learned of a single positive antigen test conducted at Manchester Medical Center on July 3. On July 10, another positive result was reported, and over the course of the week, that number grew to 59.

FILE - This undated electron microscope image made available by the U.S. National Institutes of Health in February 2020 shows the Novel Coronavirus SARS-CoV-2, the virus causes COVID-19. A new type of coronavirus test offers a cheaper, quicker way to screen for infections, moving the U.S. toward the kind of mass screening that experts say is essential to returning millions of Americans to school and work. But the first so-called antigen test _ announced Saturday, may 9, 2020 by the Food and Drug Administration _ is not quite the kind sought by top government health officials. It is less accurate than the current gold standard for testing and can only be run on specialized equipment. (NIAID-RML via AP)

Antigen testing is a newly-FDA approved method that provides rapid results. Though helpful as a screening tool for those who are symptomatic, antigen tests have demonstrated the potential for producing false negatives, according to Health Commissioner Mark Levine, and are considered less reliable than PCR (polymerase chain reaction) tests, which are typically used to diagnose the disease.

“While they’re useful, they may have a higher chance of missing an active infection,” Commissioner of Heath Mark Levine said during a media briefing Thursday.

More:Test results in Manchester point to 58 ‘presumptive positives’ for COVID-19

Those who tested positive by antigen testing were encouraged to get a PCR test, and as of July 17, 15 of the 17 confirmatory tests performed came back negative.

“This is very curious because antigen tests are much more likely to false negatives,” Levine said. 

What could explain the false positives?

Antigen tests should be used to screen symptomatic patients in populations with a high prevalence of disease, according to the Association of Public Health Laboratories. If those who were asymptomatic took the antigen tests, it could explain the false positives, and the test’s efficacy for those who are asymptomatic has not been studied, Levine said.

More:Vermont becomes the only state in the nation without a death from COVID-19 in the past 30 days

“It could turn out to be a great test for everybody, we just don’t know,” Levine said. “It’s really difficult at this point in time to understand how best to deal with these discordant results, because they are truly breaking new ground.” The state is actively seeking guidance from the CDC on the new method.

The health department does not regulate which tests individual sites perform, Levine said, and are often unaware of which method has been used until the results are reported to the state.

Manchester cases not considered an outbreak

While the presumptive positive cases from antigen testing are not included in the Department of Health’s statistics until confirmed, the state treats them in the same way they do a case confirmed by a PCR test: contact tracing is performed, and patients are advised to self-isolate and monitor for symptoms.

The cluster of presumptive positives in Manchester is not being classified as an outbreak, Levine said Thursday. An outbreak isn’t necessarily classified as such because of the number of cases, but their relationship to one another.

Those who received a positive antigen test or who have been in contact with someone who has tested positive for COVID-19 are encouraged to get tested at one of the state’s pop-up sites. Testing is available at Riley Rink in Manchester from 9 a.m. to 3 p.m., and the site will remain open for as long as necessary.