Factors associated with increased bronchial responsiveness in adolescents and young adults: the importance of adjustment for prechallenge FEV1.

Background: Asymptomatic increased bronchial responsiveness (BR) appears to be a major risk factor for later development of asthma, but risk factors for increased BR are not well established.

Objective: The relationship of age, gender, house dust mite allergy, number of blood eosinophils, and FEV1 to the degree of nonspecific BR was studied in a population sample of 665 adolescents and young adults, aged 13 to 23 years.

Methods: Case history, especially concerning smoking habits and respiratory symptoms, was obtained by interview and a self-administered questionnaire. Pulmonary function, number of blood eosinophils, BR to inhaled histamine, and skin test reactivity to house dust mites were measured with standard techniques. BR was analyzed as both a continuous variable (dose-response slope) and a categorical variable (cutoff: PC20 16 mg/ml).

Results: Increasing number of blood eosinophils, sensitivity to house dust mite, and former or current symptoms of asthma were associated with increasing levels of BR, independently of the level of FEV1. Adjustment for prechallenge FEV1 revealed a strong association between lower prechallenge FEV1 and increasing level of BR (p less than 0.0001), whereas the previously observed association between female gender and higher level of BR vanished. Repeating the analyses after exclusion of subjects with former or current asthma (n = 85) showed an unchanged relationship between house dust mite allergy and level of BR whether or not prechallenge FEV1 was included in the regression model, whereas gender only influenced the level of BR when no adjustment was made for prechallenge FEV1. On the contrary, the number of blood eosinophils was only significantly associated with BR after adjustment for baseline FEV1. Comparable findings were observed when patients with former or current symptoms of asthma and subjects with a prechallenge FEV1 less than 80% of predicted value (n = 46) were excluded from the analyses.

Conclusions: This analysis suggests that adjustment for prechallenge level of FEV1 should be considered in studies concerned with risk factors for symptomatic and asymptomatic bronchial hyperresponsiveness.