Study objectives: The exercise test is one of the most widely used challenge tests for asthma. It is not clear if the sensitivity of this test is lower when patients with asthma are not symptomatic. Since asthma activity is season dependent, with lower activity in the summer, we sought to determine if the percentage of positive exercise test results for asthma is lower in the summer as well.
Design: In this retrospective study, the proportion of positive exercise test results for asthma during each of the four seasons of the year, over a 5-year period, was compared.
Setting: The study was conducted at the pediatric respiratory clinic and pulmonary function laboratory of Shaare Zedek Medical Center.
Participants: The study group consisted of 532 consecutive patients, all 17 years of age, who were referred for exercise testing for evaluation of suspected asthma. All had normal baseline FEV1 levels, and none received maintenance asthma treatment.
Measurements and results: Spirometry was performed before and after a standard exercise regimen. A decrease of ≥ 10% in FEV1 after exercise was considered positive; 141 patients (26%) had a positive test result. The percentage of positive results in the summer (July to September quartile) was 12.9%, less than half the percentage of positive results during any of the other three quartiles (January to March, 28.9%, p < 0.05; April to June, 30.6%, p < 0.05; October to December, 29.1%, p < 0.05).
Conclusions: This study shows that the percentage of positive exercise challenge test results in the summer is half that of any other season. It is likely that the lower percentage of positive results for asthma in the summer reflects the lower sensitivity of the test during this season due to decreased asthma activity. We suggest performing exercise challenge testing only when patients are symptomatic. Physicians caring for patients with symptoms suggestive of exercise-induced asthma whose exercise test results are negative while asymptomatic should consider repeat exercise testing when their patients are symptomatic.