The diurnal variability in the peak expiratory flow rate (PEFR) can be used to determine the severity of a respiratory disease such as asthma.
The diurnal variation is the ratio of the daily change in amplitude divided by the mean value, expressed as a percent.
diurnal variation in percent =
= ((maximum PEFR) - (minimum PEFR)) / (mean PEFR) * 100%
