More than 20 years of registration of type 1 diabetes in Sardinian children: temporal variations of incidence with age, period of diagnosis, and year of birth

We analyzed Sardinian registry data to assess time trends in incidence rates (IRs) of type 1 diabetes during the period 1989-2009 (2,371 case subjects 0-14 years of age). Poisson regression models were used to estimate the effects of sex, age, period of diagnosis, and birth cohorts. IR was 44.8 cases/100,000 person-years (95% CI 43.1-46.7). The annual increase was 2.12% (1.45-2.80; test for linear trend, P < 0.001). For boys, the increasing trend was evident up to 5 years of age and for girls up to 8 years of age. Compared with the 1989-1994 birth cohort, the relative risk increased from 0.78 (0.61-1.10) in 1974-1979 to 1.62 (1.18-2.23) in 2004-2009. The increase over period was less striking, with a tendency to regress in more recent years. The best-fitting model for boys included age and a linear time trend, and for girls age and nonlinear effects of calendar period and birth cohort. In conclusion, incidence increased over time, and the increase tended to level off in more recent years by calendar period but not by birth cohort, with some evidence of a stronger increase among girls than boys. Should the increase be attributable to the effects of some perinatal environmental factor, this would mean that such a factor has started affecting females before males.