Use of peak flow variability and methacholine responsiveness in predicting changes from pre-test diagnosis of asthma.

Asthma is usually diagnosed clinically. This study investigated how methacholine challenge and peak expiratory flow monitoring influenced change from a pretest clinical diagnosis. Records of 132 patients referred with respiratory symptoms, who subsequently had reliable measurements of both airway responsiveness (provocative concentration of methacholine causing a 20% fall in forced expiratory volume in one second (FEV1 (PC20)) and peak expiratory flow variability (PEFV) were reviewed. Initial and final diagnoses for each patient were classified as: a) definite asthma; b) possible asthma; and c) definitely not asthma. The predictive value of PEFV and PC20 regarding overall change from pre- to post-test diagnosis, change from initial diagnosis of possible or definitely not asthma, and change from initial diagnosis of definite asthma, were tested by multiple logistic regression analysis. Odds ratios for PC20 were expressed per doubling dose, and for PEFV per 5% variability. Clinical diagnosis of definite asthma and definitely not asthma were confirmed in 70% and 79% respectively. PC20, but not PEFV, predicted an overall change between pre- and post-test diagnosis. Both PC20 and PEFV independently predicted change to definite asthma. PEFV and interaction between PC20 and PEFV predicted a change in those whose initial diagnosis was definite asthma. Although both measurements showed a significant correlation, there was poor agreement between positive tests. Both peak expiratory flow variability and provocative dose of methacholine causing a 20% fall in forced expiratory volume in one second influence diagnostic decision-making in patients with a high pre-test probability of asthma.