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Time Of Day, Year Matters For Asthma Testing

Medically reviewed by Carmen Pope, BPharm. Last updated on March 17, 2025.

By Dennis Thompson HealthDay Reporter

MONDAY, March 17, 2025 -- The time of day -- or year -- an asthma test takes place can affect its accuracy, a new study says.

Routine asthma testing is more reliable when done in the morning, researchers reported in the journal Thorax.

Starting at 8:30 a.m., the chances of the test accurately detecting a person’s asthma decreases by 8% for every subsequent hour that passes, results show.

People are also 33% less likely to have a positive result on an asthma test if it is done during the autumn compared to the winter, researchers found.

These results startled researchers, even though they jibe with the known fact that patients respond better to asthma drugs in the morning than in the afternoon.

“Given what we know about how the risk of an asthma attack changes between night and day, we expected to find a difference in how people responded to the lung function test, but even so, we were surprised by the size of the effect,” said lead investigator Ben Knox-Brown, lead research respiratory physiologist at Royal Papworth Hospital NHS Foundation Trust in Cambridge, U.K.

“This has potentially important implications,” he added in a news release. “Doing the test in the morning would give a more reliable representation of a patient's response to the medication than doing it in the afternoon, which is important when confirming a diagnosis such as asthma.”

A typical asthma test is a two-step process, researchers said.

First, patients are asked to take as big and deep a breath as possible through a tube, and then to blow out as hard and fast as they can. Their breath passes through a device called a spirometer, which measures lung function.

Next, the patient inhales the drug salbumatol -- a fast-acting medication that opens up the airways -- and repeats the breathing test.

If the second set of results are better than the first, that indicates that the airways must have been narrower or obstructed to begin with, suggesting that the patient has asthma.

For this study, researchers analyzed real-world data on 1,600 patients who took asthma tests at Cambridge University Hospitals NHS Foundation Trust between 2016 and 2023.

They found that the odds of having a positive response to salbumatol -- and thus, an accurate signal for possible asthma -- decreased 8% an hour as the day wore on.

People were also more likely to respond to salbumatol in the winter than in the autumn, increasing their odds of an accurate asthma diagnosis.

A combination of factors is likely behind this difference, senior researcher Akhilesh Jha, a clinician scientist at the University of Cambridge, said in a news release.

“Our bodies have natural rhythms – our body clocks,” Jha said. “Throughout the day, the levels of different hormones in our bodies go up and down and our immune systems perform differently, for example. Any of these factors might affect how people respond to the lung function test.”

Evidence of such a body-clock effect has shown up in other fields of medicine, Jha added.

“We know, for example, that people respond differently to vaccinations depending on whether they’re administered in the morning or afternoon,” Jha said. “The findings of our study further support this idea and may need to be taken into account when interpreting the results of these commonly performed tests.”

Disclaimer: Statistical data in medical articles provide general trends and do not pertain to individuals. Individual factors can vary greatly. Always seek personalized medical advice for individual healthcare decisions.

© 2025 HealthDay. All rights reserved.

Posted March 2025

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