A Discriminant Analysis Applied to Methacholine Bronchoprovocation Testing Improves Classification of Patients as Normal, Asthma, or COPD

Abstract

Two discriminant functions, incorporating baseline measurements of pulmonary function and measures of airway responsiveness, were developed to improve patient classification into groups of normal, asthma, or COPD. Accuracy of group classification was compared between the usual laboratory method (single discriminating cut-off) and these new mathematically developed functions. Forty-five normal subjects, 27 asthmatic patients, and ten well-defined COPD patients were entered into the analysis. Measurements of airway responsiveness were determined by measurement of both specific airway conductance (SGaw) and spirometry (FEV1) after sequential inhalation of methacholine. A single discriminant cut-off using measures of SGaw (PD35) or FEV1 (PD20) does not sufficiently discriminate asthma from groups that contain normal and COPD subjects (67 to 71 percent predictive value). On the other hand, our discriminant functions demonstrated improved patient classification (positive predictive value, 88 to 89 percent). We conclude that bronchoprovocation tests used to evaluate the diagnosis of asthma should incorporate measures of baseline lung function into the analysis. This, we believe, is especially necessary when baseline lung function demonstrates minimal airflow obstruction and the possibility of other causes of airway disease exist.