Influence of outdoor temperature and humidity on the methacholine challenge test

Objective of this study was to evaluate whether outdoor temperature and humidity can influence methacholine test results in outpatients living in temperate areas. 4,723 subjects (2,391 males; age: 35.1 ± 16.15; FEV1 = 100.36% [relative interquartile range (IQR):92.34-108.8]) that performed methacholine tests for suspected asthma between 2000 and 2010 were considered. Outdoor minimum, mean, and maximum temperature values (°C), relative humidity (%), and dew point (Tdp), registered when performing the tests, were examined. Airways hyperresponsive patients, with PD20 (provocative dose to obtain a 20% fall in FEV1) \3,200 lg were 2,889 (61.2%) and median PD20 was 359 lg [IQR:160-967]. On receiving operating curve (ROC) analysis, temperature, humidity, and Tdp did not significantly predict airways hyperresponsiveness (AHR), even using a 200, 800, and 3,200 lg cutoffs to identify AHR. When subjects were subdivided into subgroups, according to different levels of temperature, humidity, and dew point, no differences in PD20 and prevalence were found. Only a higher number of hyperresponsive subjects was detected in smokers when they were tested in hot and humid conditions. A weak but significantly positive relationship between PD20 and mean,maximum, and minimum temperatures was detected in severe hyperresponsive subjects (PD20\200 lg) (r = 0.100, 0.112, 0.110, respectively; p = 0.001). The regression logistic model showed that maximum temperature was a significantly protective factor for AHR (OR:0.995, 95 % CI: 0.982-0.998; p = 0.012) especially in severe hyperresponsive subjects (OR:0.988, 95 % CI: 0.977-0.999; p = 0.035). In conclusion, weather conditions do not seem to influence PD20 values obtained with methacholine tests in real life. Hot and humid environments may increase the prevalence ofAHR in smokerswhile a temperature increase may reduce the AHR risk especially in severe hyperresponsive subjects.