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%


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