Independent effects of socioeconomic status and place of residence on the incidence of childhood type 1 diabetes in Western Australia

Objective: To analyze the incidence of type 1 diabetes in 0- to 14-year olds in Western Australia, from 1985 to 2002, by region and socioeconomic status. Methods: Primary case ascertainment was from the prospective population-based Western Australian Diabetes Register, and secondary case ascertainment was from the Western Australian Hospital Morbidity Data System. The address at diagnosis was used to categorize cases into urban, rural and remote areas and into five socioeconomic groups using the Index of Relative Socioeconomic Disadvantage. Denominator data were obtained from the Australian Bureau of Statistics. Poisson regression was used to analyze the incidence rates by area and socioeconomic status. Results: There were a total of 1143 cases (904 urban, 190 rural and 49 remote). Case ascertainment was estimated to be 99.8% complete. The mean annual age-standardized incidence from 1985 to 2002 was 18.1 per 100 000 person years in urban (95% CI: 16.3-19.9), 14.3 per 100 000 in rural (95% CI: 11.4-7.3) and 8.0 per 100 000 in remote areas (95% CI: 5.8-10.3). The incidence was significantly higher in urban compared with rural (rate ratio 1.27, p = 0.001) and remote (rate ratio 2.28, p < 0.001) areas. The incidence increased with higher socioeconomic status. The incidence in the highest socioeconomic group was 56% greater than the lowest socioeconomic group (rate ratio 1.56, p < 0.001). These differences in incidence by socioeconomic status and region were independent of each other. Conclusions: Higher socioeconomic status and residence in the urban area are independently associated with an increased risk of childhood type 1 diabetes in Western Australia.