Patients tested for asthma or suspected asthma have lower lung function and are more likely to show bronchodilator responsiveness, if spirometry is conducted in the morning compared with later in the day, according to the results of the SPIRO-TIMETRY study published in the journal Thorax.¹

Researchers conducted a retrospective analysis of spirometry data from 1620 patients, aged ≥18 years, referred to Cambridge University Hospitals NHS Foundation Trust between 2016 and 2023. All the eligible participants had undergone spirometry testing before and after administration of 400 µg of salbutamol. Testing was performed using calibrated equipment, and bronchodilator responsiveness was assessed according to both the 2005 and 2022 American Thoracic Society (ATS)/European Respiratory Society (ERS) criteria. Their aim was to assess whether bronchodilator responsiveness varies based on the time of day (morning versus afternoon; 08:30–16:30) and season of testing. Most patients (58%) had been referred for spirometry for asthma or suspected diagnosis of asthma.

Bronchodilator responsiveness was identified in 25% of patients using the ATS/ERS 2005 criteria (change in FEV1 or FVC≥12%and ≥200mL of the initial value) and in 26% using the 2022 criteria (change of >10% relative to the predicted value for FEV1 or FVC). For both definitions, a higher proportion of patients tested in the morning showed bronchodilator responsiveness compared to those tested in the afternoon—28% versus 22% using the 2005 criteria, and 28% versus 23% using the 2022 criteria.

Multivariable analysis, after adjusting for age, sex, BMI, smoking history, and prebronchodilator lung function, showed that with each additional hour across the working day, starting at 8.30am, the likelihood of a positive bronchodilator response decreased by 8% (OR 0.92). A similar pattern emerged when comparing morning and afternoon testing periods with reduced odds of bronchodilator responsiveness in the afternoon (OR 0.68). When results were categorized by referral reason, the influence of time of day on the odds of having a positive bronchodilator response was observed only among patients referred for asthma or suspected asthma, while there was no association in those referred for other reasons.

Additionally, bronchodilator responsiveness was found to be more common during the winter months compared to other seasons. Compared with patients tested in winter, those tested in autumn had significantly lower odds of exhibiting bronchodilator responsiveness, but this was observed only when using the ATS/ERS 2005 definition (OR 0.67).

Asthma is typically characterized by diurnal variation in symptoms. These findings demonstrate the significance of timing in asthma diagnostics, suggesting that conducting spirometry in the morning may reliably improve the detection of bronchodilator responsiveness. The authors suggest that clinicians should not only consider the timing when interpreting spirometry results but also ensure it is documented whenever possible. Repeat testing should ideally be performed at the same time of day.

Reference

1.   Ben Knox-Brown, et al. Effect of time of day and seasonal variation on bronchodilator responsiveness: the SPIRO-TIMETRY study. Thorax. 2025 Mar 11:thorax-2024-222773. doi: 10.1136/thorax-2024-222773.