Norwegian population surveys on respiratory health in adults: objectives, design, methods, quality controls and response rates

Abstract

Background and aims: Quantifying the prevalence of asthma, chronic obstructive pulmonary disease (COPD) and restrictive pulmonary diseases in Norway is needed to document the burden of chronic respiratory inflammatory diseases on disability, health care costs and impaired quality of life. To introduce effective interventions for prevention, cure and care, there is a prerequisite to know the environmental causes. Furthermore, using relevant and precise phenotypes from community-based studies are important for detecting molecular-genetic causes for diseases.

Methods: The Norwegian Population Survey Initiative on Respiratory Health in Adults has, for four decades, applied international standardised methods for the recording of respiratory symptoms, health status, exposure to risk factors, socio-economic factors and the use of health services. Measurements of spirometry, metacholine bronchial responsiveness, transfer factor for carbon monoxide, atopy as well as chest X-ray examinations have been used advocating the internationally accepted methods. All surveys had similar quality controls, supervision and training of the field-worker team.

Results: From 1965 to 1999, random population samples, altogether including 178 690 individuals, have been invited by random sampling to seven surveys on respiratory health in the counties of Oslo (39 998 people) and Hordaland (138 692 people). The surveys were initiated in 1964, 1972, 1985, 1988, 1991 and two in 1998. The age span of those invited persons varied from 15 to 74 years at baseline. It included 43 330 women and 135 537 men. Altogether 130 075 (73%) persons participated by returning an answered questionnaire. Spirometry results are available from 41 335 persons at baseline. A biobank for DNA and blood markers has been established. Data from longitudinally clinical-epidemiological studies were available by 2007, for three surveys after 20 years, 10 years and 6-7 years, and also for parts of three other surveys, while one survey has been examined for cause-specific mortality after 30 years. The response rates of the baseline studies varied from 90% to 68% of those invited and, in general, it has declined over 35 years. The response rate of the longitudinal studies with follow-ups also declined with time after the baseline study.

Conclusions: Great challenges for future population-based studies are (i) to keep the participation rates high in community studies; (ii) to standardise the basic clinical-epidemiological methods over decades of follow-up and to systematically transfer these methods into new populations with different languages and cultures and (iii) to focus on important research questions on respiratory health for the community.