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. 2012 Jan 4;2(1):1.
doi: 10.1186/2045-7022-2-1.

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

Linnéa Hedman  1 , Anders Bjerg, Bo Lundbäck, Eva Rönmark
Affiliations

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

Linnéa Hedman et al. Clin Transl Allergy. .

Abstract

Background: 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.

Methods: In 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).

Results: The 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).

Conclusion: the 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.

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Figures

Figure 1
Figure 1
Flow chart of the identification of the population at risk for asthma and wheeze based on annual questionnaire surveys.
Figure 2
Figure 2
A: Annual incidence rates of physician-diagnosed asthma from age 12 to18 years by sex, based on annual reports. B: Annual incidence rates of current wheeze from age 12 to 18 years by sex, based on annual reports.
Figure 3
Figure 3
Cumulative incidence (%) of asthma and wheeze from age 12 to 18 years based on annual reports (AR) and the baseline and endpoint surveys only (BE), respectively.
Figure 4
Figure 4
The distribution of asthma severity among incident cases of current asthma identified in the annual reports (AR) and based on the baseline and endpoint surveys only (BE), respectively.
See this image and copyright information in PMC

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