Health care experts believe one-third of patients with the novel coronavirus and who are tested for it receive negative test results—meaning some people who are infected could be spreading the virus unknowingly.

FDA fast-tracks tests

According to the Wall Street Journal, all lab tests have a margin of error, as determined by the test's sensitivity—the percent of positive cases that the test correctly assesses—and its specificity, or the percentage of negative cases that it correctly assesses.  

Currently, the New York Times reports that the most common method for detecting the new coronavirus relies on a process known as the reverse transcription polymerase chain reaction, or RT-PCR, which can usually detect small amounts of viral RNA. And according to Tom Taylor, a professor and former statistician at CDC, the agency in a normal situation would conduct a study to determine the accuracy of the tests. However, that research could take up to a year.

In light of growing demand, FDA relaxed requirements that would ordinarily have required testing manufacturers to submit applications for the new tests, allowing labs to start using the tests without formal approval and to submit their internal research on the tests at a later date.

Ultimately, Bill Miller, a physician and epidemiologist at the Ohio State University, said, "The whole testing field is in flux." He explained that the majority of the tests are "going through a really rapid validation process," due to increasing demand, and "[a]s a result we can't be completely confident in how they will perform."

Are the tests inaccurate?

Overall, Taylor estimated the polymerase chain reaction (PCR) tests correctly detect more than 90% of positive cases. However, based on the available information, Mike Lozano, an executive at Envision Healthcare, estimates the sensitivity of currently available tests appears to be lower than comparable tests—likely around 70%, which means nearly one-third of patients who have the virus receive a false negative.

And in February, a study published in Radiology found that of 1,000 patients in Wuhan who were suspected to have the coronavirus, 60% tested positive for the virus even though 90% showed signs of the condition in CT scans of their chest.

According to Chris Smalley, a primary care physician (PCP) at Norton Healthcare, that 70% estimate seems accurate based on real-world experiences, absent any formal data.

For instance, several doctors—including Smalley—have reported that patients showing symptoms similar to Covid-19 who received negative test results went on to become hospitalized with worsened symptoms. Some of these patients even received a positive test result on a later date.

For example, Peter Brokish, an ED doctor in Dallas, said multiple tests he sent to LabCorp came back negative, but the patients later returned to the ED with worsened symptoms. After being hospitalized, their tests came back positive.

"You kind of figure, yeah, that's a false negative," Brokish said.

A spokesperson for LabCorp, however, said that test accuracy could also be impacted by how the specimen was collected or transported, as well as how long the person was infected with the virus before testing. For instance, according to the Journal, a swab taken from the front of the nose may not be as accurate, as those taken from intersection of the nasal cavity and throat.

"A negative result does not definitively rule out infection," the spokesperson said.

According to the Journal, the problem could be exacerbated by the lack of testing kits. For instance, few doctors are willing to re-test a patient who already tested negative unless they have to be hospitalized for their symptoms.

What happens when someone gets a false negative?

According to Craig Deligdish, an oncologist at Omni Healthcare, "A false negative is problematic because it tells the patient they don't have the virus." In turn, patients who receive a negative result will likely behave as if they aren't ill, Deligdish said, which could cause people with false negative results to spread the new coronavirus unknowingly.  

This was the case for Michele Hickle from Texas, who fell ill with a fever and cough on March 17. Because of her symptoms, Hickle assumed she had the virus, so she isolated herself from her family. However, when she received her results from a drive-thru testing facility later that week, the results came back negative. "At that point, we welcomed her back into the family," her stepson, Scott Hickle, said.

But Hickle's symptoms worsened. She went to an urgent care center to get tested again, and that time the test was positive. Now Hickle is concerned she passed the virus to her family—noting that her daughter Grace is now showing symptoms.

Linda Girgis, a PCP in New Jersey, said there is a similar concern among health care workers, who could come to work assuming that they are not carrying the virus and, therefore, are not contagious.

Ultimately, health experts say if you've been exposed to Covid-19 or have similar symptoms, you should assume you have it. Dara Kass—an ED doctor at NewYork-Presbyterian, who is working remotely after testing positive for the virus last month—added that clinicians should assume that patients with symptoms have Covid-19, especially in hotspots where the disease is widespread.

"A negative is not clearing anybody who is symptomatic," she said (Weaver, Wall Street Journal, 4/2; Krumholz, New York Times, 4/1).