Age-period-cohort analysis of 1990-2003 incidence time trends of childhood diabetes in Italy: the RIDI study.

OBJECTIVE: to investigate age-period-cohort effects on temporal trend of type 1 diabetes in age 0-14 years in Italian registries. RESEARCH DESIGN AND METHODS: This report is based on 5180 incident cases in the period 1990-2003. Multilevel (random intercept) Poisson regression models have been used to model the effects of sex, age, calendar time and birth cohorts on temporal trends taking into account the registry-level variance component. RESULTS: The incidence rate was 12.26 per 100,000 person-year, significantly higher in boys (13.13; 95% confidence interval (CI): 12.66-13.62) than in girls (11.35; CI: 10.90-11.82). Large geographical variations in incidence within Italy were evident, being highest in Sardinia, intermediate in Central-Southern Italy, and high in Northern Italy, particularly in the Trento Province, where an incidence rate of 18.67/100,000 was registered. An increasing temporal trend was evident (2.94% per year, CI 2.22-3.67). With respect to the calendar period 1990-92, the incidence rates increased linearly, being 15%, 27%, 35% and 40% higher in the following time periods (p for trend <0.001). With respect to the birth cohort 1987-1993, the incidence rate ratio increased approximately linearly from 0.63 (CI 0.54-0.73) in the cohort 1975-81 to 1.38 (CI 1.06-1.80) in the cohort 1999-2003. The best model, however, was the one with sex, age and a linear time trend (drift). CONCLUSIONS: Large geographical variations and an increasing temporal trend are evident in Italy. Age period-cohort analysis shows that the variation over time has a linear component that cannot be ascribed to either the calendar period or the birth cohort.