Why a Negative Test Doesn't Rule Out COVID

Brenda Goodman, MA

October 05, 2020

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

White House press secretary Kayleigh McEnany's positive COVID-19 test raises more concerns about relying on tests to rule out the disease.

McEnany released a statement on Twitter saying she had tested negative consistently, including every day since Thursday. She said she would go into quarantine immediately.

McEnany did not say what type of test she had taken to confirm her diagnosis. The White House has reportedly relied on the Abbott ID Now test in the past, which it uses to screen visitors and the president daily.

Rapid tests like ID Now are convenient, and they can return results in as little as 15 minutes, but they are not as accurate as the gold-standard PCR tests, which require lab processing and return results in 1-2 days.

The FDA warned in May that the test could return false negative results. Studies have found that the test misses between 1 in 3 and 1 in 7 of positive cases.

The FDA advised that anyone testing negative on the ID Now have their results confirmed with a different testing method. It's unclear whether the White House has been doing this confirmatory testing.

Whatever test is used, they can miss COVID cases for a variety of reasons, says Isaac Bogoch, MD, an infectious disease specialist at the Toronto General Hospital Research Institute.

"Sometimes the sample isn't collected properly. Sometimes there's a problem with the device, and someone might be positive but not shedding the virus yet. Those are some reasons that come to mind," he says.

After the virus that causes COVID-19 enters the body, it has to penetrate our cells and corrupt their operating instructions so it can churn out more copies of itself to infect more cells and make more copies. It takes a few days for this copying to happen enough to be detected on a test.

For COVID, it takes about 3 to 5 days after infection for someone to make enough copies to be picked up by a test. There appears to be a small window, maybe as short as hours, when a person is shedding the virus and infectious, but doesn't yet have enough virus in their body to be detected by a less sensitive rapid test. If a person is tested while they are in this window, their case would be missed.

That also means that more people who attended the Sept. 29 presidential debate may test positive in the coming days. At least 11 people involved in the event have tested positive for COVID-19, also raising questions about how the virus could have run rampant there despite precautions, which included testing for everyone who attended.

The Cleveland Clinic supervised the event. In a statement issued Friday, it said it had requirements to keep a safe environment that aligned with CDC guidelines – including social distancing, hand sanitizing, temperature checks, and masking. "Most importantly, everyone permitted inside the debate all tested negative for COVID-19 prior to entry. Individuals traveling with both candidates, including the candidates themselves, had been tested and tested negative by their respective campaigns."

"They're not out of the woods," Bogoch says, even if they tested negative at first, as Biden has. For that reason, he says anyone who's been exposed to COVID should follow CDC guidelines to quarantine themselves for at least 14 days.

WebMD Health News


News release, city of Cleveland, Oct. 2, 2020.

Isaac Bogoch, MD, clinical investigator, Toronto General Hospital Research Institute.

Journal of Clinical Microbiology: "Performance of Abbott ID Now COVID-19 Rapid Nucleic Acid Amplification Test Using Nasopharyngeal Swabs Transported in Viral Transport Media and Dry Nasal Swabs in a New York City Academic Institution."

News release, FDA, May 14, 2020.

Twitter, @PressSec, Oct. 5, 2020.