COMPARISON OF REPORTED PREVALENCES OF RECENT ASTHMA IN LONGITUDINAL AND CROSS-SECTIONAL STUDIES

A potential source of bias in prevalence rates reported for symptoms a nd diagnoses of asthma in longitudinal studies could arise if repeated questioning of subjects or previous experience of lung function and a irway responsiveness tests increased awareness of respiratory symptoms . We wished to determine the extent of any such bias by comparing repo rted prevalence rates from a longitudinal and cross-sectional study wi thin similar populations. The prevalences of wheezing in the last year , waking with chest tightness, waking with shortness of breath, waking with coughing, having an attack of asthma in the last year, and curre nt use of medications for asthma were determined using identical quest ions in two populations, Self-completed questionnaire responses of 946 subjects, 21 yrs of age, participating in the seventh respiratory ass essment in the longitudinal Dunedin Multidisciplinary Health and Devel opment Research Study were compared with responses provided by 991 sub jects, aged 20-22 yrs, completing a postal questionnaire on one occasi on only for the New Zealand section of the European Community Respirat ory Health Study. The prevalence rates were not significantly differen t between the two populations, for all of the reported symptoms and fo r medication use, Differences in responses between genders were simila r in each study, with all responses being more common in females. We c onclude that repeated questioning regarding respiratory symptoms and r epeated lung function and bronchial challenge testing in a asthma did not bias prevalence rates compared with those population of the same a ge studied on only one occasion. (C)ERS Journals Ltd 1997.