Operational definitions of asthma in studies on its aetiology.

The most popular way to define asthma based on questionnaires is to use definitions taken from cross-sectional international studies on asthma. These definitions may not, however, be optimal for future studies focusing on risk factors of asthma. The current authors, therefore, compared the performance of different operational definitions of asthma. The European Community Respiratory Health Study I was a cross-sectional study of 21,924 subjects aged between 25-44 yrs in 18 countries. Operational definitions of asthma compared included different combinations of symptoms of asthma and bronchial hyperresponsiveness. A continuous asthma score, ranging from 0-8, was defined as the sum of positive answers to eight main symptom questions. There was no threshold in the associations of asthma symptoms with severity or risk factors of asthma, which would have suggested a dichotomous definition of asthma. Using dichotomous definitions requiring the presence of several asthma symptoms strengthened associations with studied risk factors, and also increased the estimated specificity and positive predictive value. Using a continuous asthma score also improved the power of the analyses. In conclusion, dichotomous definitions of asthma yielding higher odds ratios are achieved by requiring positive responses to several questions on symptoms. However, symptoms of asthma are possibly best analysed as a continuous asthma score.