Objective: Recently, self-reported asthma was combined with reported age of onset to investigate the disease's natural history. To assess the validity of reported results, we investigated the reliability of the method.
Study design and settings: The European Community Respiratory Health Survey was a longitudinal study with interviews in 1991/93 and in 2000/02. Lifelong asthma and age of asthma onset were assessed through self-administered questionnaires. Responses of 10,933 participants in the follow-up were combined to separate true from false incident cases. The repeatability of questions was assessed and the bias in cumulative incidence (CI) estimation was quantified.
Results: Age at onset had excellent reliability (mean difference between the two interviews=-0.20, weighted kappa=0.88) allowing the differentiation of false and true incident cases. Given this information, lifelong asthma question's reliability was very high (agreement=0.96, kappa=0.83). Misclassified subjects had respiratory conditions similar to the asthmatics. Baseline asthma was underreported and, if ignoring the onset age, the CI was severely overestimated (observed 5.82%, actual 3.02%).
Conclusion: Questionnaire-based longitudinal studies make more reliable estimates possible when all the retrospective information is used: the reported age of onset plays a key role and should be accounted for when investigating the natural history of diseases.