Health & Wellness

A Negative COVID-19 Test Is No Guarantee

The most common COVID-19 test produces a lot of false negatives


The test most commonly used to detect COVID-19 infections often produces false negative results, researchers from Johns Hopkins University in Baltimore warn.

A study published in the Annals of Internal Medicine found that RT-PCR tests have a high false negative rate, especially for those in the earlier stages of infection. The highest number of false negatives was found among those in the first few days of having the disease, who had yet to show symptoms. The odds of receiving a false negative ranged from 100% on the first day of infection and 67% on the fourth. On the fifth and eighth days of contracting the virus, when symptoms began showing, false negatives fell to 38% and 20%, respectively. As the days went on and symptoms began fading again, the rates of false negatives rose again.

Taken together, the results mean that RT-PCR tests produce negative results for one in five people tested, even if they have the virus.

“The important takeaway is that we know the false negative rate is very high,” Lauren Kucirka, a resident physician at Johns Hopkins and a lead author of the study, told the media.

“The focus should really be on ruling in positive patients rather than ruling out patients on the basis of the negative tests.”

Why the tests produce so many false negatives is still being determined by researchers at the university, though Kucirka said it could be due to tests being administered improperly, or it might be that patients aren’t shedding enough infected particles for tests to detect the presence of the virus. That’s why it’s important that doctors consider other factors when determining probable cases, such as whether a patient has been living or working around those with the virus.

“We know we’re probably going to miss some cases if we use this as the gold standard to diagnose cases because we know it’s not perfect,” she also said.

”It’s very important to use clinical judgment and not treat the test as a definitive answer.”

Photo: iStock/anyaivanova.