- The subject must demonstrate a diurnal variation in PEF of more than 20% based on the difference between the prebronchodilator (before taking albuterol/salbutamol) morning value and the postbronchodilator value (after taking albuterol/salbutamol) from the evening before, expressed as a percentage of the mean daily PEF value. Note: If a subject is to qualify using diurnal variation, the subject should be instructed to perform his/her PEF evaluation after using his/her bronchodilator in the evening.