July 6, 2020

What does a negative coronavirus test really mean? (Maybe not what you think.)

Daily Briefing

    While a negative coronavirus test result may come as a relief to some, health experts warn that testing inaccuracies and research limitations mean a negative test result might not actually prove you're not infected.

    The 3 biggest questions about Covid-19 testing, answered

    Questions abound over coronavirus tests' accuracy

    The most common method for detecting whether a person is infected with the new coronavirus relies on a process known as the reverse transcription polymerase chain reaction, or RT-PCR, which usually can detect small amounts of viral RNA in a biological sample.

    According to Tom Taylor, a professor and former statistician at CDC, the agency under normal circumstances would conduct a study to determine the accuracy of such tests. However, that research could take up to a year—and when pressed with expanding America's capacity to test people for the new coronavirus amid the country's growing epidemic, CDC didn't research the tests as robustly as it typically would.

    In addition, FDA in response to growing demand for the tests relaxed regulations that ordinarily would have required testing manufacturers to submit applications for the new tests and go through FDA's typical approval process. The change allowed labs to start marketing and using the tests in the United States without going through FDA's formal review process, and instead requires the labs to submit internal research on the tests' safety and accuracy at a later date.

    As a result, Bill Miller, a physician and epidemiologist at Ohio State University, said the majority of the tests were "going through a really rapid validation process" and, "[a]s a result, we can't be completely confident in how they will perform."

    Mike Lozano, an executive at Envision Healthcare, in April estimated that, based on information available at the time, the sensitivity of available coronavirus tests appeared to be around 70%, which would mean that nearly one-third of patients who have the virus would receive a false-negative result.

    Chris Smalley, a primary care physician at Norton Healthcare, at the time said that 70% estimate seemed accurate based on real-world experiences, absent any formal data. For instance, several doctors—including Smalley—had reported that patients showing symptoms similar to Covid-19 who received negative test results went on to become hospitalized with worsened symptoms, and some of those patients received a positive test result on a later date.

    A study published last month in Annals of Internal Medicine in May suggested the PCR tests were slightly more accurate, missing about 20% of positive coronavirus cases.

    Further, experts have expressed concerns that the tests may not detect coronavirus infections in the early days after a person was first exposed to and contracted the pathogen. Bob Wachter, chair of UCSF's Department of Medicine, said it can take between three and five days after exposure for a test to be able to detect infection.

    The tests also may be less accurate in people who have contracted the new coronavirus but are not experiencing symptoms of Covid-19, the disease caused by the virus. Emily Landon, a hospital epidemiologist and infectious diseases specialist at University of Chicago Medicine, said, "We don't know how good these tests are in individuals who don't have symptoms." She explained, "We know they're pretty good at picking up [the virus] when it's present in people who have symptoms. But we have no idea what a negative test means in an individual that doesn't have symptoms."

    What do you do if you test negative?

    In light of the issues regarding testing accuracy, health experts warn that receiving a negative test result doesn't necessarily mean a person isn't infected with new coronavirus.

    FDA cautions that "[n]egative results must be considered in the context of an individual's recent exposures, history, presence of clinical signs and symptoms consistent with Covid-19."

    Steven Woloshin, co-director of Dartmouth Institute's Center for Medicine and Media, said people who are experiencing no symptoms and are "at low risk because [they] live in some remote area, [they're] practicing social distancing, [and they] always wear a mask" likely can assume their negative test result is a true negative. However, he said, "If you have symptoms or you work in a place where you're at high risk for exposure, then even with a negative test, you might want to think really hard about it."

    Landon explained that researchers "are certain that there are people who test negative" for the virus "even though they are definitely contagious." She noted, "A positive test can make us relatively certain that you are shedding [the new coronavirus]. But a negative test does not mean the opposite."

    And because of that issue, people should continue to practice social distancing even if they receive a negative result, experts urge—especially because testing negative one day doesn't mean you won't contract the new coronavirus the next.

    "A week from now, that test that you had today that was negative means virtually nothing," Wachter said, adding that people who test negative should continue to social distance and use face masks or coverings to protect themselves against possible transmission (Arcuni, KQED, 6/19; Pandey, NewsBytes, 6/22; Wamsley, "Goats and Soda," NPR, 5/31; Zhang, The Atlantic, 6/21).

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