Accuracy in Testing
The Health Department considers false negative COVID-19 test results a bigger public health threat than false positives.
Inaccurate COVID-19 test results are rare, according to the director of the Knox County Health Department, and the small number of false positive tests providers have uncovered are not inflating the county’s case count.
Doctors are reporting that the testing has shown three or four false positives for every 1,000 true positives.
Dr. Martha Buchanan said Friday that no test is 100 percent accurate but that the PCR test the Health Department uses is the most reliable test available.
“There are many factors that can contribute to false negatives and false positives, everything from specimen collection to how many viral particles have to be present for a test to be positive,” she said.
A false positive test is one that indicates the novel coronavirus is present in a patient who actually has not been infected; likewise, a false negative result indicates a patient is illness-free even though the novel coronavirus is actually present.
The issue of false positive tests is at the heart of a lawsuit filed by two plastic surgeons against Buchanan, Knox County, the Health Department and Dr. Jack Gotcher Jr., chair of the Board of Health. Drs. Steven J. Smith and Jason J. Hall contend the county’s mandate to wear masks in most public buildings violates their rights.
“Defendants relied on faulty data which includes false positives for the support and adoption of Regulation No. 2020-1,” the complaint alleges.
The complaint cites a July 6 U.S. Food and Drug Administration letter to laboratories and health care providers warning that one manufacturer’s reagents result in false readings in 3 percent of the positive test results.
While Buchanan did not address the lawsuit directly during her Friday news briefing, she did explain that the Health Department, following state Department of Health guidelines, treats all people who test positive as having COVID-19 out of abundance of caution.
“We don’t want the implication to be that false positives are ballooning our case numbers because that’s not the case,” she said. “There is thought to be much higher false negative reporting. That means there are more people in our community who have the virus, but the test doesn’t show that.”
A false positive result would require a person to needlessly go into quarantine. False negatives, however, are more of a community threat than false positives, Buchanan said.
“From a public health perspective, false negatives are actually more of a concern because that means someone could be returning to life as normal — gathering with their family and friends and going to work — when they are actually positive for COVID-19,” she said. “That could lead to further transmission of this virus, as they can spread it without even knowing they are positive.”
The FDA letter cited in the lawsuit recommends that all positive test results be treated as presumed positive cases: “Consider any positive result presumptive from tests using the BD SARS-CoV-2 Reagents for the BD Max System. Consider confirming with an alternate authorized test.”
Buchanan said that local infectious disease doctors are reporting that the testing has shown three or four false positives for every 1,000 true positives. That would mean approximately 16 of the 3,997 positive test results reported through Sunday would be the maximum number of false positives possible, which assumes all the positive results have come via PCR tests.
“That’s a pretty good ratio,” Buchanan said.
One factor that can lead to false negatives is that COVID-19 isn’t always detectable immediately after exposure. Getting tested too soon can lead to a negative result and a false sense of security, Buchanan said.
“If someone waits a few days after they’ve been exposed to get a COVID-19 test, it’s more likely to be positive if they’re going to develop COVID-19,” she said.
According to Buchanan, a few people have gotten multiple tests for a variety of reasons and have received both positive and negative results.
“For those tests, it would not be accurate to refer to them as false positives, because it is not known which of the two tests is incorrect,” she said. “Therefore, you can't assume the positive is the wrong one.”
Buchanan asked that people should not get a second test while waiting for the results of their first one.
“There’s no need to get another test because it might come back faster,” she said. “You can’t test your way out of quarantine.”
Testing is a diagnostic tool and should not be used as a substitute for the five core actions — wearing a mask, practicing social distancing, washing hands, cleaning surfaces and staying home when sick — that public health officials recommend for slowing the spread of the novel coronavirus, Buchanan said. Having multiple negative tests is no protection.
“Remember, the test is not prevention,” she said. “The five core actions are prevention.”