| Summary: | Abstract Background Exercise‐induced bronchoconstriction (EIB), a strong positive predictor of asthma, becomes progressively less frequent with age. Although asthma tends to become less common only in boys during adolescence, sex differences in EIB, especially in preschoolers, remain unclear. To find EIB for early diagnosis and intervention asthma, mass‐screening tests considering sex differences in preschoolers are needed. In this study, we investigated whether sex differences influence the prevalence and severity of EIB in prepubertal children aged 5–6 years. Methods Fifty‐one children aged 5–6 years who were attending a kindergarten in Matsuyama City, Ehime, Japan, were enrolled in this cross‐sectional study. The children underwent a 6‐minute free‐running test in 2015. The peak expiratory flow rate (PEFR) was measured before exercise and 0, 3, 10, and 20 minutes after exercise. The severity of EIB was classified according to the reduction in PEFR, measured as the difference between the postexercise PEFR and the highest pre‐exercise PEFR. Results Of the 51 children (23 boys and 28 girls) enrolled, the prevalence of EIB defined as three criteria: a ≥15%, ≥20%, or ≥25% decrease was 54.9% (28/51), 41.2% (21/51), and 25.5% (13/51), respectively. The prevalence of EIB defined as ≥25% decrease was significantly higher in girls than in boys (39.3% vs 8.7%, P = .013). In girls, the mean percentage change in PEFR was significantly higher 20 minutes than 10 minutes postexercise (P = .043). Conclusions Sex difference in the prevalence and severity of EIB should be considered when evaluating EIB, even in young, prepubertal children.
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