Asthma Knowledge Centre
Assessment
Peak expiratory flow measurements
This information is taken from the revised GINA Report, Global Strategy for Asthma Management and Prevention (2007), available on the Global Initiative for Asthma (GINA) website. Please refer to the full guideline document for more detailed information.
Peak expiratory flow measurements are made using a peak flow meter and can be an important aid in both diagnosis and monitoring of asthma. Modern PEF meters are relatively inexpensive, portable, plastic, and ideal for patients to use in home settings for day-to-day objective measurement of airflow limitation. However, measurements of PEF are not interchangeable with other measurements of lung function such as FEV1 in either adults1 or children.2 PEF can underestimate the degree of airflow limitation, particularly as airflow limitation and gas trapping worsen. Because values for PEF obtained with different peak flow meters vary and the range of predicted values is too wide, PEF measurements should preferably be compared to the patient’s own previous best measurements3 using his/her own peak flow meter. The previous best measurement is usually obtained when the patient is asymptomatic or on full treatment and serves as a reference value for monitoring the effects of changes in treatment.
Careful instruction is required to reliably measure PEF because PEF measurements are effort-dependent. Most commonly, PEF is measured first thing in the morning before treatment is taken, when values are often close to their lowest, and last thing at night when values are usually higher. One method of describing diurnal PEF variability is as the amplitude (the difference between the maximum and the minimum value for the day), expressed as a percentage of the mean daily PEF value, and averaged over 1-2 weeks.4 Another method of describing PEF variability is the minimum morning pre-bronchodilator PEF over 1 week, expressed as a percent of the recent best (Min%Max) (Figure 2-2).4 This latter method has been suggested to be the best PEF index of airway ability for clinical practice because it requires only a once-daily reading, correlates better than any other index with airway hyperresponsiveness, and involves a simple calculation.
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To confirm the diagnosis of asthma. Although spirometry is the preferred method of documenting airflow limitation, a 60 L/min (or 20% or more of prebronchodilator PEF) improvement after inhalation of a bronchodilator,5 or diurnal variation in PEF of more than 20% (with twice daily readings, more than 10%6) suggests a diagnosis of asthma.
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To improve control of asthma, particularly in patients with poor perception of symptoms.8 Asthma management plans which include self-monitoring of symptoms or PEF for treatment of exacerbations have been shown to improve asthma outcomes.7 It is easier to discern the response to therapy from a PEF chart than from a PEF diary, provided the same chart format is consistently used9
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To identify environmental (including occupational) causes of asthma symptoms. This involves the patient monitoring PEF daily or several times each day over periods of suspected exposure to risk factors in the home or workplace, or during exercise or other activities that may cause symptoms, and during periods of non-exposure.
References:
1. Sawyer G, Miles J, Lewis S, Fitzharris P, Pearce N, Beasley R. Classification of asthma severity: should the international guidelines be changed? Clin Exp Allergy 1998;28(12):1565-70.
2. Eid N, Yandell B, Howell L, Eddy M, Sheikh S. Can peak expiratory flow predict airflow obstruction in children with asthma? Pediatrics 2000;105(2):354-8.
3. Reddel HK, Marks GB, Jenkins CR. When can personal best peak flow be determined for asthma action plans? Thorax 2004;59(11):922-4.
4. Reddel HK, Salome CM, Peat JK, Woolcock AJ. Which index of peak expiratory flow is most useful in the management of stable asthma? Am J Respir Crit Care Med 1995;151(5):1320-5.
5. Dekker FW, Schrier AC, Sterk PJ, Dijkman JH. Validity of peak expiratory flow measurement in assessing reversibility of airflow obstruction. Thorax 1992;47(3):162-6.
6. Boezen HM, Schouten JP, Postma DS, Rijcken B. Distribution of peak expiratory flow variability by age, gender and smoking habits in a random population sample aged 20-70 yrs. Eur Respir J 1994;7(10):1814-20.
7. Gibson PG, Powell H. Written action plans for asthma: an evidence-based review of the key components. Thorax 2004;59(2):94-9.
8. Killian KJ, Watson R, Otis J, St Amand TA, O'Byrne PM. Symptom perception during acute bronchoconstriction. Am J Respir Crit Care Med 2000;162(2 Pt 1):490-6.
9. Yssel H, Abbal C, Pene J, Bousquet J. The role of IgE in asthma. Clin Exp Allergy 1998;28 Suppl 5:104-9.