Peak flow variability calculator

Peak Expiratory Flow Rate (PEFR) in patients with asthma is usually lower in the morning and higher in the evening.  This is called diurnal variation.

The calculator below will work out your patient's diurnal variability percentage, using the Daily Amplitude Percent Mean Method.

The daily mean percentage column shows you the variability of each day as a percentage.  

These scores are then added up and divided by the number of days, to give you the overall diurnal variability score.  

Diurnal variation of 10 - 20% is suggestive of asthma.

For accurate results:

• the PEFR should be recorded at least twice a day, on waking and ideally around 4pm when peak flow tends to ‘peak’.  
• the patient should perform at least 3 blows each time, and record the highest number . 
• the diary should be kept for 2-4 weeks.  

Enter a minimum of 7 days of peak flow readings and click the calculate button.  

Click on "Print this page" to download or print the calculated results to add to your patient's notes.