Nebraska doctors discuss COVID-19 ‘false-negative’ rate

LINCOLN — More than 7,400 Nebraskans have been tested for COVID-19 at hospitals, urgent care clinics and drive thru clinics.  Over 6,900 have come back negative, those thousands of Nebraskans celebrating the diagnosis. But new concerns are growing in some medical circles that some negative results, might not be accurate.

“I think it’s possible that you could have up to a 30 percent false-negative rate, it’s also possible it’s a bit less than that, maybe it’s 10 percent, maybe it’s 20 percent,” Bryan Health Medical Director of Infection Prevention Dr. Jim Nora said. “The important thing is it’s probably not zero.”

Bryan Health’s Dr. Jim Nora was the most cautious of the three experts interviewed about the rate of false-negatives.  The concerns stem from a Chinese study published in a collection of preliminary medical reports. The study, and it’s estimation of a 30 percent false negative rate, was cited last week in a Yale doctor’s New York Times editorial and an article in the Boston Globe.

In Nebraska, the director of the Nebraska Public Health Lab says it’s not time to throw away the test.

“The answer is no. First off, the sensitivity is pretty good, it’s probably greater than 90 percent,” Dr. Pete Iwen said. “When you say a negativity of 30 percent, I would say it’s really dependent on where the sample came from, it’s dependent on the pre-analytical processing of how the sample is handled. It’s also dependent on the particular assay itself.”

Dr. Pete Iwen also pointed out the Chinese study hasn’t been peer reviewed and the preferred nasal-pharyngeal test wasn’t used.

There are possibilities for error, though. Dr. Iwen says mistakes could include using the wrong swab, swabbing incorrectly, improper labeling, transporting samples in the wrong temperature, and taking too long to test the sample.  His lab rejects about one percent of samples because they’re incorrect.

Another possible false-negative trigger, is that patients have stopped shedding virus.

“There may be only a short window of opportunity to do testing,” Dr. Nora said. “It may be just a few days, maybe it’s seven days… we don’t know at this point.”

But get past the pre-analytical phase, and CHI Infectious Disease Dr. David Quimby is confident in the results.

“Information from the company puts it at 95 percent or greater sensitivity, which means you would have very, very few false negatives,” Dr. Quimby said. “When we did in-house testing with the known samples, our results were somewhat better than that.”

The doctors all agreed, if you’re still showing symptoms, a negative test result shouldn’t change your behavior.

“Just what you should do when you have any sort of respiratory infection, don’t pass it along to other people,” Dr. Quimby said. “Stay at home, isolate yourself, good hand hygiene, good cough etiquette, those types of things.”

The experts say concerns don’t apply to positive results. They haven’t heard of any false positives. But apply the possible five percent false negative rate to Nebraskans, and that’s more than 340 false negatives.

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