Effect of time of day and seasonal variation on bronchodilator responsiveness: the SPIRO-TIMETRY study

We investigated the association between time of day and season of testing on the level of bronchodilator responsiveness in a hospital-based population. We found that per 1-hour increment in the working day, the odds of a positive bronchodilator response decreased by 8%. A similar effect was seen when time of day was dichotomised into morning and afternoon time periods. When stratifying by referral reason, the impact of time of day was only seen in those referred for asthma/query asthma. We also found that bronchodilator responsiveness was more common in winter months compared with the rest of the year.