A NEW study highlights the significant impact of time of day and season on bronchodilator responsiveness, suggesting that asthma diagnostic testing may be more effective when conducted in the morning.
Researchers retrospectively analysed spirometry data from 1,620 patients referred to Cambridge University Hospitals NHS Foundation Trust between 2016–2023. Eligible participants were adults who underwent spirometry testing before and after inhaling 400 µg of salbutamol. Spirometry was performed using calibrated equipment, with bronchodilator responsiveness defined according to both the 2005 and 2022 American Thoracic Society (ATS)/ European Respiratory Society (ERS) criteria. Statistical models assessed the association between testing time, season, and the likelihood of bronchodilator responsiveness while adjusting for age, sex, BMI, smoking history, and prebronchodilator lung function.
Findings showed that 25–26% of patients exhibited bronchodilator responsiveness, depending on the diagnostic criteria used. Morning tests yielded a higher proportion of positive responses than afternoon tests under both ATS/ERS definitions (28% vs 22% for 2005; 28% vs 23% for 2022). Multivariable analysis confirmed that with each passing hour in the working day, the odds of bronchodilator responsiveness declined by 8% (odds ratio [OR]: 0.92, 95% CI: 0.88–0.97), with a significant reduction in the afternoon (OR: 0.68, 95% CI: 0.54–0.85). Patients referred for asthma or suspected asthma were particularly affected by this variation, with a significant interaction observed between referral reason and time of testing (p<0.001). Seasonal differences were also evident, with the highest bronchodilator responsiveness occurring in winter, and significantly lower odds in autumn (OR: 0.67, 95% CI: 0.48–0.92).
These results reinforce the importance of timing in asthma diagnostics, suggesting that morning spirometry may enhance the likelihood of detecting bronchodilator responsiveness. Clinicians should consider test timing when interpreting spirometry results, and future studies should investigate repeated testing across different times of day.
Reference
Knox-Brown B et al. Effect of time of day and seasonal variation on bronchodilator responsiveness: the SPIRO-TIMETRY study. Thorax. 2025;DOI:10.1136/thorax-2024-222773.
