Study: The Time of Day You Get Tested for COVID Matters, Too

When people get tested for COVID-19, getting accurate results is crucial. Research has already established that when you get tested after exposure to the virus can impact how reliable your results are. But now, new research suggests there could be one more element to consider when getting tested: the time of day.

Researchers analyzed data from 86,342 clinical tests performed in people who were symptomatic and asymptomatic for COVID-19 at a regional healthcare network in the southeastern U.S. from March to August 2020, specifically looking at the time people were tested.

Researchers found there was up to a 1.7-fold variation in the portion of tests that were positive for SARS-CoV-2, the virus that causes COVID-19, based on time of day. The peak for positive results was 2 p.m.  The results were published in the Journal of Biological Rhythms in October.

“These findings have important implications for public health testing and vaccination strategies,” the researchers wrote.

The data, the researchers said in a press release, support the idea that COVID-19 acts differently in the body based on circadian rhythm, or the natural bodily processes that regulate the body’s internal clock. COVID-19 appears to release more infectious particles into the blood and mucus in the middle of the day because the immune system is impacted by our biological clock, they point out. 

“Taking a COVID-19 test at the optimal time of day improves test sensitivity and will help us to be accurate in diagnosing people who may be infected but asymptomatic,” study co-author Carl Johnson, PhD, a professor of biological sciences at Vanderbilt University, said in a statement.

How Accurate Are COVID Tests?

The gold standard for determining a COVID-19 infection is a polymerase chain reaction (PCR) test.

These tests are usually performed in a lab and involve making copies of the virus’ genetic materials, Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo in New York, tells Verywell.

“These tests are considered very accurate,” he says.

The exact accuracy of PCR tests is tough to pin down given that research has found varying results. One study analyzed data from 95,919 patients in Canada who were tested for COVID-19 and found that, of them, only 49 produced results that were inaccurate.

But, while PCR tests are highly accurate, they aren’t perfect. Research shows that when a person gets tested during the course of illness can help determine the accuracy of results.

An analysis published in the Annals of Internal Medicine in August 2020 looked at data from 1,330 COVID-19 tests and found that infected people are 100% likely to receive a false-negative test result the day after they’re exposed and only 33% likely to get an accurate reading by day four.

By the time the patient develops symptoms, test results are about 62% accurate, rising to about 80% accuracy eight days after they were infected. As a result, the researchers concluded, it’s best to get tested for the virus about eight days after exposure. 

You Shouldn't Wait to Get Tested

It’s important to note that this particular study didn’t determine that testing results were more accurate at certain times of the day. The researchers found that tests were more likely to be positive around the early afternoon.

But experts say that COVID-19 tests should be able to accurately detect an infection at any time—when they’re used properly.

“COVID tests, in general, are accurate but it’s important to reflect on the question you’re asking of the test: ‘What am I sick with?’ or ‘Am I contagious?’” Amesh A. Adalja, MD, infectious disease expert and a senior scholar at the Johns Hopkins Center for Health Security, tells Verywell. “There may be a circadian influence on viral shedding, but many negative tests may be due to the fact that testing occurs later in the illness.” 

Russo agrees that “there could be some differences in viral shedding based on time of day.”

"[But] certainly the PCR test should be sensitive enough so that you could be tested pretty much any time of day and, if you're truly positive, you should get a positive test result," he adds.

If you need to get tested for COVID-19, Russo doesn’t recommend waiting until 2 p.m. on any given day to do so.

“If you want to get a test, go get tested when you can,” he says. “You don’t need to time it out.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.