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.
Measurements of lung function. The diagnosis of asthma is usually based on the presence of characteristic symptoms. However, measurements of lung function, and particularly the demonstration of reversibility of lung function abnormalities, greatly enhance diagnostic confidence. This is because patients with asthma frequently have poor recognition of their symptoms and poor perception of symptom severity, especially if their asthma is long-standing1. Assessment of symptoms such as dyspnea and wheezing by physicians may also be inaccurate. Measurement of lung function provides an assessment of the severity of airflow limitation, its reversibility and its variability, and provides confirmation of the diagnosis of asthma. Although measurements of lung function do not correlate strongly with symptoms or other measures of disease control in either adults2 or children3, these measures provide complementary information about different aspects of asthma control.
Various methods are available to assess airflow limitation, but two methods have gained widespread acceptance for use in patients over 5 years of age. These are spirometry, particularly the measurement of forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC), and peak expiratory flow (PEF) measurement.
Predicted values of FEV1, FVC, and PEF based on age, sex, and height have been obtained from population studies. These are being continually revised, and with the exception of PEF for which the range of predicted values is too wide, they are useful for judging whether a given value is abnormal or not.
The terms reversibility and variability refer to changes in symptoms accompanied by changes in airflow limitation that occur spontaneously or in response to treatment. The term reversibility is generally applied to rapid improvements in FEV1 (or PEF), measured within minutes after inhalation of a rapid-acting bronchodilator—for example after 200-400 mg salbutamol (albuterol)4—or more sustained improvement over days or weeks after the introduction of effective controller treatment such as inhaled glucocorticosteroids.4 Variability refers to improvement or deterioration in symptoms and lung function occurring over time. Variability may be experienced over the course of one day (when it is called diurnal variability), from day to day, from month to month, or seasonally. Obtaining a history of variability is an essential component of the diagnosis of asthma. In addition, variability forms part of the assessment of asthma control.
References:
1. 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.
2. Kerstjens HA, Brand PL, de Jong PM, Koeter GH, Postma DS. nfluence of treatment on peak expiratory flow and its relation to airway hyperresponsiveness and symptoms. The Dutch CNSLD Study Group. Thorax 1994;49(11):1109-15.
3. Brand PL, Duiverman EJ, Waalkens HJ, van Essen-Zandvliet EE, Kerrebijn KF. Peak flow variation in childhood asthma: correlation with symptoms, airways obstruction, and hyperresponsiveness during long-term treatment with inhaled corticosteroids. Dutch CNSLD Study Group. Thorax 1999;54(2):103-7.
4. Pellegrino R, Viegi G, Brusasco V, Crapo RO, Burgos F, Casaburi R, et al. Interpretative strategies for lung function tests. Eur Respir J 2005;26(5):948-68.