Socioeconomic position and participation in baseline and follow-up visits: the Inter99 study

Background: The aim of this paper was to identify the extent of socioeconomic inequality in participation at baseline and follow-up visits. Design: The Inter99 study is a randomized intervention with the aim of investigating the effects of an individualized lifestyle consultation on ischaemic heart disease (IHD). The study comprised 61,301 persons of which 13,016 were assigned to the intervention group. The rest formed the control group. All those in the intervention group were invited to participate in health examinations, risk assessments, and lifestyle consultations. Participants at high risk of IHD were invited to follow-up visits after 1, 3, and 5 years. Methods: Data on five socioeconomic factors were retrieved from nationwide registers. For each socioeconomic factor we estimated the relative risks and relative index of inequality of participation at the baseline visit and among high-risk participants at follow-up visits. In addition, we conducted analyses of trends in socioeconomic inequality in participation across follow-up visits. Results: Participation rates were 53% at baseline and 61-65% at the three follow-up visits. There was strong socioeconomic inequality in participation at baseline, with increasing probability of participation found with increasing level of socioeconomic position. This was smaller at follow-up visits. Except for education and housing tenure, there was an increase in socioeconomic inequality in participation across follow-up visits. Conclusions: We found strong socioeconomic inequality in participation at baseline and follow-up visits. Effort should be made to increase participation in individualized lifestyle interventions among persons of low socioeconomic position. Otherwise, the consequence may be increased socioeconomic inequality in IHD.