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What Mussels Can Teach Us About False Positive COVID-19 Tests


Many scientists who usually have nothing to do with viruses or infectious disease are turning their attention to COVID-19. For example, one wildlife biologist is raising questions about the accuracy of tests that detect the coronavirus. NPR's science correspondent Richard Harris has that story.

RICHARD HARRIS, BYLINE: In normal times, Andrew Cohen focuses his attention on issues of ecology and conservation.

ANDREW COHEN: My specialty is in the area of biological invasions, and I work on both marine and freshwater organisms.

HARRIS: A few years ago, the state of California hired him to fight back the invasion of non-native mussels, which had been wreaking havoc on ecosystems in the eastern United States.

COHEN: We began getting reports after that of these mussels showing up all across the western U.S.

HARRIS: Scientists were using a clever technique to find them. They take a water sample and then look for the tiniest traces of genetic material from these mussels. They used a test called PCR, which vastly amplifies genetic material to look for mussel DNA. And they kept getting positive results.

COHEN: And I began to realize that many of these, if not all of these, were false positives.

HARRIS: Cohen wanted to understand why these tests were going awry, so he could spread the word to the labs that were using them.

COHEN: I eventually turned to the medical literature to look at assessments that had been done of medical diagnostic labs that used PCR-based testing in humans. And I found that there was a pretty good false positive rate in those tests.

HARRIS: PCR false positives averaged about 2%, depending upon the lab and the test. Fast forward now to March 2020, and Cohen starts thinking about this when he is puzzled about reports of people who test positive for the coronavirus with PCR tests but have absolutely no symptoms.

COHEN: I began wondering if these asymptomatic carriers weren't in large part or perhaps in whole part the human counterparts of those false positive results of quagga and zebra mussels in all those water bodies across the West where, actually, no one has ever found a quagga or zebra mussel. It turned out that they were, in fact, all false positives.

HARRIS: So he and a colleague posted a paper online outlining the results of his survey of the medical literature. It hasn't been peer reviewed, but Dr. Bobbi Pritt, who chairs the committee concerned about these issues for the College of American Pathologists, says his numbers seem reasonable.

BOBBI PRITT: There is no 100% perfect test.

HARRIS: A PCR test itself may produce no false positive results, but errors inevitably creep in as the tests are used in the real world. People mishandle samples. Labels get switched. And PCR is so incredibly sensitive, contamination is a particular concern. Even the tiniest amount of stray material in a lab can spell trouble, Pritt says.

PRITT: Viral material could get into the environment and then contaminate your specimens around you and cause false positive results in those specimens.

HARRIS: Even the automated testing machines, like the one at the White House, can produce false positive results. They use simple plug-in cartridges.

PRITT: It doesn't mean that they are foolproof. We've observed some of these tests, and these little cartridges have issues, such as leaking even amplified viral nucleic acid into the laboratory, which can cause quite a bit of problem.

HARRIS: The College of American Pathologists is now starting to help labs evaluate how well they're doing running PCR tests for the coronavirus. And there's a lot at stake for getting these tests right. People can be sent to isolation for weeks with a positive test result. And false positives can also skew our knowledge of where the disease is spreading. That's especially a problem in areas where there are very few true positives. As Andrew Cohen found with the mussels, false positives in that situation can actually overwhelm the results. Richard Harris, NPR News.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

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