A positive methacholine challenge based on specific airway conductance: A case report.

A 30-year-old Caucasian man presented to the pulmonary function laboratory for a methacholine challenge test. Following inhalation of the final dose of methacholine, the forced expiratory volume in 1 s (FEV1) was 8% below baseline. However, the patient complained of chest tightness and dyspnea, similar to the symptoms he experienced after running. Repeat specific airway conductance was found to be 73% below baseline, indicating marked airway hyper-responsiveness. Because the reduction in specific airway conductance was accompanied by familiar symptoms, the post-test probability of asthma increases, even in the absence of a 20% reduction in FEV1.