Conventional epidemiology underestimates the incidence of asthma and wheeze‐a longitudinal population‐based study among teenagers

Linnéa Hedman,Bo Lundbäck,Anders Bjerg,Eva Rönmark

doi:10.1186/2045-7022-2-1

Abstract

BackgroundBecause of shifts in the gender ratio and incidence and remission rates of asthma during the teen ages, the methodology of incidence studies among teenagers is important, i.e. if the time intervals between surveys are too long, the incident cases might not be properly identified. The aim was to study the impact of study design on the incidence rates of asthma and wheeze during the teen ages.MethodsIn a study about asthma and allergic diseases within the OLIN studies (Obstructive Lung Disease in northern Sweden), a cohort of school children (n = 3,430) was followed annually by questionnaire from age 8 yrs. In the endpoint survey (age 18 yrs) 2,582 (75% of original responders) participated. Incident cases from age 12-18 yrs were identified by two methods: annual questionnaire reports (AR) and baseline-endpoint surveys only (BE).ResultsThe cumulative incidence of asthma and wheeze was significantly higher based on AR compared to BE. Compared to the incidence rates based on all the annual surveys, the calculated average annual rates based on BE were in general lower both among the boys and among the girls. There were no differences between boys and girls in incidence rates of asthma or wheeze during the early teen years. However, from the age of 15 years, the annual incidence rates were significantly or borderline significantly higher among girls than boys. At onset, the additional cases of current asthma identified by AR had significantly less severe asthma than those identified in BE (p < 0.02).Conclusionthe size of the incidence of asthma and wheeze during the teen ages was influenced by study design. By using the conventional prospective study design with longer follow-up time, the incidence was underestimated.

Highlights

Because of shifts in the gender ratio and incidence and remission rates of asthma during the teen ages, the methodology of incidence studies among teenagers is important, i.e. if the time intervals between surveys are too long, the incident cases might not be properly identified
Annual incidence rate Based on annual questionnaire reports (AR), there were no differences between boys and girls in incidence rates of physician-diagnosed asthma or current wheeze during the early teen years (Figure 2a and 2b)
Compared to AR, the analyses based on the incident cases identified by baseline-endpoint surveys only (BE) yielded similar but slightly higher odds ratios and wider confidence intervals

Summary

Introduction

Because of shifts in the gender ratio and incidence and remission rates of asthma during the teen ages, the methodology of incidence studies among teenagers is important, i.e. if the time intervals between surveys are too long, the incident cases might not be properly identified. The aim was to study the impact of study design on the incidence rates of asthma and wheeze during the teen ages. The number of population-based studies about the incidence of asthma and wheeze during adolescence are still limited [1,2,3,4,5,6]. In the majority of these studies, an average annual incidence rate was estimated based on two surveys, several years apart. Despite considerable differences in the methodology and definitions, the available studies show that the incidence of asthma is highest in early childhood and decreases in adolescence. The incidence of asthma has been reported at 0.6-1.1/100 per year in pre-teenagers [7] and teenagers [1,2,8]. Compared to most of these prospective studies, lower incidence rates have been reported both in retrospective [10,11] and register studies [12]

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