Study shows that, even if infected, people tested for Sars-CoV-2 in the first four days are 67% more likely to have negative results.
Researchers at Johns Hopkins University in the United States concluded that testing people for Covid-19 at the beginning of the infection increases the chances of getting a false negative result. The scientists published an article describing the finding in May, in the journal Annals of Internal Medicine.
One of several ways to assess whether or not a person is infected with the new coronavirus is through a method called reverse transcription polymerase chain reaction (RT-PCR). This test looks for traces of the genetic material of Sars-CoV-2 in the body of the person being tested.
However, as shown in tests with other microorganisms, such as the flu virus, if the sample is collected incorrectly or if the viral load is very low in the person’s body, the tests can produce false negatives. “A negative test, whether or not a person has symptoms, doesn’t guarantee that they aren’t infected by the virus,” said Lauren Kucirka, a resident doctor and lead author of the research, in a statement to the press. “How we respond to, and interpret, a negative test is very important because we place others at risk when we assume the test is perfect.”
The researchers compared the test results with the time it took for people to test positive for Covid-19. According to the team, it is estimated that those tested for Sars-CoV-2 in the four days after infection were 67% more likely to have negative results, even though they were infected with the virus.
Among patients who had the new coronavirus and had symptoms of infection, the rate of false negatives was 38%. The test performed best eight days after infection (on average, three days after the onset of symptoms). Still, a 20% false-negative result rate was observed.
With this in mind, Kucirka believes that health professionals must pay attention mainly to patients’ symptoms. She further points out that “as we develop strategies to reopen services, businesses and other venues that rely on testing and contact tracing, it is important to understand the limitations of these tests,” she said.