Vanderbilt Study: Accurate positive COVID-19 result twice as likely if tested in middle of day compared to at night

Coronavirus

NASHVILLE, Tenn. (WKRN) — With many people getting tested for COVID-19 before seeing family for Thanksgiving, there’s new research about how the time of day impacts test results.

“The answer was really clear that there’s a two-fold difference between the day and night and that the positivity rate is more likely to get a positive test during the day than at night,” said Carl Johnson, Cornelius Vanderbilt Professor of Biological Sciences.

At the beginning of the pandemic, Professor Johnson said he wondered how the virus might act differently depending on the time of day and the body’s circadian rhythms.

“I have a good friend of mine, who’s the first author on the paper, Dr. Candace McNaughton, and she’s an emergency room physician,” said Professor Johnson. “One of the things that, in discussions with her right at the kind of beginning of the COVID crisis, we were talking about this and she mentioned that people who are COVID positive, that their symptoms tend to be worse at some times a day than others. And of course, my ears perked right up about that, you know, that’s what I’m always interested in.”

They looked at about 85,000 tests done by Vanderbilt that were time-stamped. While most were obviously during the day, a significant amount was at night from inpatient tests and the emergency room. The researchers found that people were up to two times as likely to have an accurate positive test result if they tested in the middle of the day compared to at night. It has to do with what’s called “virus shedding,” which is when infected cells release infected virus particles into the blood and mucus. Researchers say this appears to be more active in the middle of the day due to the modulation of the immune system by our biological clock.

“That was consistent with the idea that maybe the virus is getting shed into our blood or mucus or whatever, you know, early in the day, so that it’s fairly high there in the day, and then it kind of decreases during the night. So that’s not absolutely proven until we do a direct test, but it certainly is consistent with that,” said Professor Johnson.

This leads to the other likelihood of the virus spreading more during the day since infected cells are shedding more during that time as well.

“The key thing is, it doesn’t mean that we can go out to a bar at 11 o’clock in the night and then sneeze in people’s faces and not worry about it, it’s just as worrisome from that perspective,” he explained.

Ever since the discovery of COVID-19, researchers have been working to find out more about the virus so they can help policymakers make the best decisions about how to handle the pandemic.

“There are the implications in terms of how we extend this to patient care and things like that. And I think the good news of our study was that the optimal time of testing is during the day, so, fortunately, that kind of fit,” he said. “If we got a different result, say, for example, that, you know, the optimal time was two o’clock in the early morning or something like that, and during the daytime, the test wasn’t as sensitive, that could have caused a major shift in what we might try to do. Now, we’re reassured that that’s not necessary. But we wouldn’t have done that if we hadn’t done the testing, do the experiments and analysis beforehand.”

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