A negative coronavirus test doesn't necessarily mean you're in the clear for COVID-19. It turns out timing has a lot to do with the accuracy of test results.
In a seven-study analysis, a team of researchers from Johns Hopkins University found that patients who were tested with the most common coronavirus test (known as an RT-PCR test) soon after becoming infected with the virus were more likely to receive a false-negative result compared with those who were tested when symptoms of COVID-19 appeared, or shortly thereafter. A false negative is when the test results indicate a person is free of the virus, even if that isn't the case.
The most accurate time to take the test was two to three days after symptom onset, when the likelihood of receiving a false-negative result dropped significantly, according to the study, published in the Aug. 18 issue of Annals of Internal Medicine. However, even on the days the test worked best, about 1 in 5 infected people still tested negative for the coronavirus.
"A negative test doesn't guarantee you don't have the virus. It means you may not be infectious at that very moment when you were tested,” explains Justin Lessler, one of the study's authors and an associate professor of epidemiology at Johns Hopkins Bloomberg School of Public Health.
"But that can change quickly,” he adds. And in the meantime, the virus can spread to others.
When to get a coronavirus test
If you're displaying common symptoms of COVID-19, get tested immediately, Lessler says. Symptoms usually appear two to 14 days after exposure to the virus and can range from fever and fatigue to new loss of taste or smell, according to the Centers for Disease Control and Prevention (CDC).
If you have been exposed to someone with COVID-19, but are not showing symptoms, you should still get tested, experts say — just not right away. “Wait two to three days because the virus needs time to replicate,” Lessler advises.