Education And Debate

Diurnal variability—time to change asthma guidelines?

BMJ 1999; 319 doi: https://doi.org/10.1136/bmj.319.7201.45 (Published 03 July 1999) Cite this as: BMJ 1999;319:45
  1. Helen Reddel, research scholar (hkr@mail.med.usyd.edu.au),
  2. Christine Jenkins, head of clinical trials,
  3. Ann Woolcock, director
  1. Insitute of Respiratory Medicine, Royal Prince Alfred Hospital and University of Sydney, PO Box M77, Camperdown, NSW 2050, Australia
  1. Correspondence to: Dr Reddel
  • Accepted 26 March 1999

Peak expiratory flow varies throughout the day in normal subjects, and this diurnal variation is increased in people with asthma.1 Current asthma guidelines recommend that diurnal variability of the peak expiratory flow rate should be calculated when diagnosing asthma and assessing its severity,27 including during exacerbations.34 Diurnal variability of peak flow has been used as a marker of airway responsiveness,8 9 particularly in epidemiological studies,10 11 and as an outcome measure in clinical asthma trials.12 However, there are problems associated with its use.

Summary points

Variation in peak flow over days or weeks provides helpful information about asthma control

Asthma guidelines recommend that diurnal peak flow variability is calculated to provide an index of airway lability

These calculations are too time consuming for normal clinical practice

Factors such as the time of recording or recent use of β2 agonist drugs result in minor changes in peak flow, but can cause large errors in diurnal variability

Diurnal variability may fail to detect important changes in lung function

An alternative, simpler index of peak flow variation such as the lowest morning peak flow expressed as percentage of the patient's personal best peak flow value should be evaluated for inclusion in asthma guidelines

Cumbersome calculations

Although diurnal variability in peak flow has been included in asthma guidelines for many years, doctors in primary and secondary care settings rarely use it, because of the cumbersome calculations involved. Several alternative equations may be used. The most common are the amplitude percentage mean ((maximum—minimum)/mean) or the amplitude percentage maximum ((maximum—minimum)/maximum), calculated for each day, and then averaged over a period of 1 to 2 weeks.13 Determining the amplitude percentage mean from as few as 7 days of twice daily peak flow readings for one patient (see fig 1) is …

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