David A. Kaminsky

Background and Objective Airway hyperresponsiveness (AHR) is commonly assessed by a methacholine challenge test (MCT), during which a provocative concentration causing a 20% reduction in forced expiratory volume in 1 second (FEV 1 ) (PC 20 ) < 8 mg/ml is considered a positive response. However, a fall in specific airway conductance (sGaw) may also have clinical significance. The purpose of this study was to assess whether AHR determined by a provocative concentration causing a 40% reduction in sGaw (PC 40 ) < 8 mg/ml corresponds to a clinical diagnosis of asthma. Methods We analysed the changes in spirometry, lung volumes and sGaw during MCT in 211 randomly selected patients being evaluated for AHR to support a clinical diagnosis of asthma. Results The mean (SD) age of the group was 53 (15) years, with 141 women (67%). Overall lung function was normal, with FEV 1 = 92 (15) % predicted, total lung capacity = 97 (13) % predicted and sGaw = 0.19 (0.15–0.23) L/s/cm H 2 O/L, (median, 25–75 IQR). There were many more patients who responded by PC 40 only ( n = 120) than who responded by PC 20 ( n = 52). There was no significant difference in asthma diagnosis between the PC 20 (98%) and PC 40 (93%) groups, and we estimate 34% of patients with a diagnosis of asthma would have been classified as having no AHR if only the FEV 1 criterion was used. Conclusion Changes in sGaw during MCT indicate clinically significant AHR in support of a clinical diagnosis of asthma among patients being evaluated for asthma.