False test results another challenge of new virus

By , Daily Memphian Published: March 24, 2020 4:00 AM CT

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As thousands get tested here, one of the unspoken worries is the number of false negatives, either because the infection wasn’t full blown when the test was taken, the swabbing wasn’t deep enough (it’s uncomfortable and people pull away) or the tests are inconsistent.

The result is that many people think they have received good news and won’t know the truth until days down the road when the list of people on their contact list could be bigger.

“The tests are very, very rarely positive in a negative person,” said Dr. Jon McCullers, associate dean of the UTHCS College of Medicine and pediatrician in chief at Le Bonheur Children’s Hospital. “It is much more likely to get a negative in an infected person.”

Last weekend, trainers from the University of Tennessee Health Science Center “spent a good deal of time training” at the drive-up testing center on Tiger Lane where the university is expecting to test thousands before the siege is over.

Most of the tests there will be administered by medical students. The challenge is getting the swab deep enough in the nasal cavity to get a good sample to identify if the virus is there.

One of the issues is that the primers used to detect the viral genome do not match the virus very well, McCullers said, which throws off results. And if the swabs were stored at an incorrect temperature, the result can be erroneous. Ditto on procedures themselves in the labs where the specimens are tested.

And there is likely to be variations in the tests themselves, although it’s all so new, none of that has been looked at yet, McCullers said.

“One of the problems we have with very rapid medical progress,” said Dr. Stephen Threlkeld, infectious disease expert at Baptist Memorial Health Care, “is that we don’t have the luxury of a year’s worth of data to calculate the exact sensitivity and specificity of the positive-predictive value and the negative-predictive value of each test.”

Because there is a “lack of clarity” about how each test performs, researchers and clinicians are now having to rely on results from the same molecular-level tests in related viruses.

The good news, Threlkeld says, is that those tests historically have done well in identifying both who has the virus and who does not.

In medical diagnosis, test sensitivity is a measure of how effective a test is at identifying people with disease, or what clinicians call a true positive rate. Specificity is how accurately a test identifies people who do not have the infection.

“We hope these are going to perform as well, but we still have some jury out on the precise performance of those individual tests,” Threlkeld said.

But no one knows. For people who have been incorrectly told they are negative, the news will be revealed soon enough, Threlkeld says, underscoring the importance of the new behaviors people are trying to abide by in their lives, increasingly being lived out with others at home.

“We have to be social distancing as aggressively as we can,” he said. “We know that will prevent us being as stressed as some of the other cities in America that didn’t have the quite the warning we have had and the opportunity to prevent it.”

Because test results can be thrown off in what happens when the sample is being delivered to the lab, Baptist is working with American Esoteric Laboratories (AEL), its longtime partner, to test solutions for transport, including saline solution, which is cheap and easy to get.

“They are working on validating those tests under various conditions,” he said.

Swabs are a real factor both because they are in short supply around the nation and because swabs can affect test results.

“Cotton swabs, for example, seem to interfere with the test,” Threlkeld said. “We need to have a non-cotton swab. That is just one example of what we are talking about.”

Because AEL in Memphis was able to ramp up production quickly last week, it is now able to process thousands of specimens per week.

That has allowed Baptist to test more people, Threlkeld said, and get a better sense of the percentage testing positive.

“We need those data in the community to make informed decisions about how we maneuver in the face of this epidemic.”

Hospital administrators across the region are using the daily uptick as a barometer for the days and weeks ahead.

“We’re following the numbers like everyone else,” said Michael Ugwueke, CEO of Methodist Le Bonheur Healthcare.

“The more tests you do, the more who will test positive. What we don’t know is how many of them will be sick enough to be in the hospital.”


Michael Ugwueke Dr. Jon McCullers Dr. Stephen Threlkeld Methodist Le Bonheur Healthcare Baptist Memorial Health Care American Esoteric Laboratories
Jane Roberts

Jane Roberts

Longtime journalist Jane Roberts is a Minnesotan by birth and a Memphian by choice. She's lived and reported in the city more than two decades. She covers business news and features for The Daily Memphian.


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