Seasonal variability of the methacholine challenge test

Background. Since the intensity of some variables that influence bronchial hyper-responsiveness (BHR) varies across different seasons of the year, the percentage of positive metacholine challenge test (MCT) is theoretically subjected to seasonal variation. This study has looked at the seasonal variability of MCT among young adults with suspected asthma. Methods. The number of MCT performed and its corresponding proportion of positive results were retrospectively evaluated and compared for different seasons over a 5-year period. Results. Overall 645 subjects (mean age 21.4 ± 2.3 yr) underwent MCT during the study period. Two-hundred nine MCTs (34.2) were performed in the winter, significantly higher when compared to any other season of the year. Positive MCT (defined as PC20 < 16 mg/mL) was noted in 163 patients (25.2). The proportion of positive MCTs has shown significant seasonal trend, (p < 0.0001, chi-square test for trend), it was significantly higher in the winter and in the spring (32.2 and 32.4, respectively) when compared with the summer (13.4). Conclusions. This study showed a significant seasonal variability for both pattern of referral to MCT and the seasonal prevalence of BHR young adults. These findings probably reflect seasonal variation in the intensity of external stimuli that involve in the pathogenesis of BHR, such as allergens burden and respiratory tract infections across the year. We suggest that interpretation of a positive MCT as predictive of BHR and of asthma should take into account the observed seasonal variability of the test.