European birth cohort studies on asthma and atopic diseases: II. Comparison of outcomes and exposures--a GA2LEN initiative.

Background: The Global Allergy and Asthma European Network (GA2LEN) is a consortium of 26 leading European research centres committed to establish a European research area of excellence in the field of allergy and asthma.
Aim: One of the GA2LEN work packages was designed to identify and compare the existing European birth cohort studies on asthma and atopic diseases. The present review compares their subjective and objective outcomes as well as exposure variables.
Methods: A common database was established to assess study characteristics of observational birth cohort studies designed to examine asthma and atopic diseases. Data were collected by visiting most of the participating research teams and interviewing all relevant study personnel. For each study, the type of objective/subjective outcome parameters and potentially influential factors were recorded precisely for every time point during follow-up.
Results: Eighteen birth cohort studies on asthma and atopic diseases were identified in eight European countries. Thirteen studies collected data on specific immunoglobulin E (IgE) to various inhalant and food allergens, whereas 12 performed skin prick tests (many at several time points during follow up). Several studies measured lung function, but across the cohorts no comparable standard procedures were used. For subjective evaluation of asthma and allergic rhinitis most studies applied the ISAAC questionnaire (sometimes modified), whereas the assessment of eczema was rather heterogeneous across the studies.
Conclusion: This GA2LEN initiative established a unique common database of 18 European birth cohorts on asthma and atopic diseases. For selected cohorts, it seems that pooling data and performing common analyses may be possible to examine associations between certain exposure variables (e.g. pet ownership, tobacco smoke exposure and day-care) and selected outcome measures for atopy, asthma or allergic rhinitis (e.g. sensitization assessed by IgE or skin prick tests, doctor's diagnosis of asthma, parental perception regarding asthma/wheezing or hay fever symptoms).