En español | 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.
That said, it's important to follow proper preventive measures during that waiting window: Stay home and away from others as much as possible to avoid spreading the virus in case you are infected. Experts still aren't certain when people with coronavirus infections are most contagious but suspect it ranges from a few days before COVID-19 symptoms appear till a few days after they start.
The CDC recently revised its guidelines to say that close contact with someone who has COVID-19 doesn't necessarily require a test “unless you are a vulnerable individual” or unless a health care expert recommends it. Many public health experts, however, disagree. If you have a question about whether you should be tested, reach out to your doctor or local health department for advice.
What to do if your test comes back negative
A negative test is not a free pass to throw caution to the wind, experts warn.
"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. However, those infected with the virus are still able to potentially spread the virus,” Lauren Kucirka, M.D., a medical resident at Johns Hopkins Medicine and lead author on the study, said in a statement.
Even with a negative test result, the CDC recommends staying home and away from others for 14 days if you were in contact with someone who has COVID-19. This helps prevent people from unknowingly spreading the virus to others, and especially to individuals who are at greater risk for getting severely ill from an infection.
If your initial test came back negative and you go on to develop symptoms of COVID-19, consider getting retested. It may turn out that your first test was conducted too soon, when not enough virus was present to generate a positive result, or that the sample was collected incorrectly. Another reason to take a second COVID-19 test is if you are unable to stay in quarantine the full two weeks.
What to do if your test comes back positive
The advice isn't much different if you test positive for the coronavirus.
Scott Weisenberg, M.D., medical director for travel medicine and an infectious disease specialist at NYU Langone Health in New York City, says patients who test positive for COVID-19 should check with their physician and local health department to find out how long to self-isolate. The CDC also has guidelines that suggest a self-isolation period of at least 10 days since symptom onset.
Keep track of any symptoms you develop, and be on the lookout for emergency warning signs. Trouble breathing, persistent pain or pressure in the chest, new confusion, inability to stay awake and blueish lips or face warrant immediate medical attention.
Delay in accurate results elevates need for frequent testing
Many public health experts warn that testing people several days after they are infected with the virus, even if that is when results are most accurate, does not help efforts to contain the spread of the virus.
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"Because we don't do enough frequent testing, we're missing people at peak,” Michael Mina, M.D., assistant professor of epidemiology at Harvard T.H. Chan School of Public Health, told reporters on a recent media call.
By the time most people get tested, they've likely already transmitted the virus to others, meaning “we caught them too late,” adds Mina, who has long called for rapid, over-the-counter home coronavirus tests, similar to a pregnancy test. Rapid tests (also called antigen tests) have a higher likelihood of false-negative results. However, if they are easily accessible and inexpensive, individuals could test themselves frequently and self-isolate more quickly if an infection is identified.
Until new testing strategies and technologies are widely available to the public, the Johns Hopkins researchers caution that “care must be taken in interpreting RT-PCR tests” for a coronavirus infection, and that “if clinical suspicion is high, infection should not be ruled out” on the basis of test results alone.
"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,” Kucirka said.