Spirometric tests to diagnose suspected asthma.

Variation in airway obstruction can occur during a single physiologic study in many asthmatic patients that distinguishes them from patients with bronchitis and from normal subjects. When this variation is expressed as a change in the one-second forced expiratory volume, (FEV1), the quantity of 15 per cent or more of the predicted FEV1 can be used to identify the asthmatic patients. This degree of change is useful in interpreting both the response to inhaled bronchodilators in obstructed patients and the lability of air flow produced by exercise and bronchodilators in nonobstructed patients. A smaller change does not exclude the diagnosis of asthma but, when such a change is measured, the suspected diagnosis of asthma is confirmed.