Normal Range of Methacholine Responsiveness in Relation to Prechallenge Pulmonary Function
Methacholine challenge testing is currently being used as a diagnostic test for a variety of clinical indications. In patients presenting with chronic cough of uncertain cause, challenge testing has been advocated as a means to determine whether the cough is associated with airway hyperresponsiveness (“cough variant asthma”) and likely to respond to bronchodilator therapy. Methacholine challenge testing also has a role in the evaluation of workers with possible occupational asthma for purposes of diagnosis or disability determination. my canadian pharmacy.com
Clinical interpretation of challenge tests performed for these indications requires the establishment of a normal range of challenge test results. Distinguishing normal from abnormal airway responsiveness is complicated by the overlapping ranges of responsiveness displayed by normal (asymptomatic, nonasthmatic) and abnormal (asthmatic) subjects. Such overlap is a feature of most measurements used as diagnostic tests, and the selection of a “cut-off’ between normal and abnormal generally involves a tradeoff between diagnostic sensitivity and specificity.
Another aspect of bronchial challenge testing that adds complexity to the establishment of a normal range is the relationship between measurements of airway responsiveness and the level of pulmonary function prior to bronchial challenge. Responsiveness to methacholine or histamine has been observed to be inversely related to level of pulmonary function (ie, greater responsiveness observed in persons with lower levels of pulmonary function) in population samples’ and among patients with COPD. Some reports have indicated a similar relationship among subjects with asthma,’ but other investigators have observed no relationship between responsiveness and prechallenge pulmonary function among asthmatics.
Previous descriptions of the relationship between airway responsiveness and prechallenge level of pulmonary function have not provided normal ranges of methacholine airway responsiveness for specific levels of pulmonary function, limiting the clinical usefulness of these reports. In this study, we examined the distribution of methacholine airway responsiveness according to level of prechallenge FEVj in a sample of asymptomatic middle-aged and elderly men with normal pulmonary function. The use of FEVj-specific criteria for abnormal PD20FEVj was compared with the use of a single lower limit of normal PD20FEVj in terms of sensitivity and specificity for detecting questionnaire-reported asthma and wheezing in a larger sample that included men with respiratory symptoms.