Methacholine challenge test.

Methacholine challenge has been proposed as a diagnostic test in situations where clinical asthma is suspected but reversible airway obstruction cannot be demonstrated spirometrically. Methacholine challenge was used in 14 patients with suspected asthma and five normal controls. Eight individuals (six patients and two controls) had a reduction in the forced expiratory volume at 1 s (FEV1) of 20% or more with 100 inhalation units (IUs) of methacholine; seven additional individuals (six patients and one control) met one or more of the American Thoracic Society's criteria for a positive bronchoprovocation test; and four individuals (two patients and two controls) were not sensitive to methacholine by any criteria. The diagnosis of asthma can be excluded if methacholine does not induce reversible obstruction. However, provocation of reversible obstruction with methacholine is not specific for asthma. More stringent criteria, such as reduction of FEV1 by 20% by 20 IUs of methacholine, would be less sensitive but more specific for asthma.