Gender differences in asthma prevalence may depend on how asthma is defined

BACKGROUND: Asthma may be defined either as wheeze within the previous 12 months (current wheeze) doctor-diagnosed asthma (DDA), or current wheeze plus confirmed airway hyperresponsiveness (AHR). AIMS: We wanted to estimate asthma prevalence in randomly selected adolescents based on different criteria for asthma diagnosis, study gender differences in reported asthma-like symptoms vs DDA, and relate our findings to measurements of AHR, levels of exhaled nitric oxide (ENO) and total IgE. METHODS: As part of the health survey of North-Trondelag (HUNT), 8571 adolescents aged 13-19 years were investigated with an interview on allergic and respiratory symptoms (phase I study). Of these, 401 subjects who reported wheeze within the previous 12 months (current wheeze) and 213 non-symptomatic controls were randomly selected and investigated with allergy screening, methacholine bronchoprovocation test and measurements of ENO (phase II study). RESULTS: In the phase I study prevalence of current wheeze was 26% (30% in girls and 23% in boys, P < 0.01). Prevalence of DDA was 10.8% (10.5% in girls and 11% in boys). Among subjects with current wheeze, the likelihood of having DDA was reduced in girls compared to boys, odds ratio (95% CI) 0.82 (0.68-0.98) which was partly explained by a longer history of wheeze among boys. In the phase II study, although more girls than boys with current wheeze had AHR (62% versus 50%, P < 0.02) more boys than girls reported DDA (44% vs. 32%, P < 0.02). Of the objective parameters, increased levels of ENO most strongly increased the risk of having DDA. CONCLUSIONS: When asthma is defined as DDA, there is a risk of underestimating the prevalence of asthma, especially among girls.