Aims/hypothesis: We reassessed the validity of previously reported incidence rates for type 1 diabetes in 0-34 year olds in Sweden. We estimated new incidence rates through three nationwide registers.
Methods: We used capture-recapture methods to assess ascertainment in the Diabetes Incidence Study in Sweden (DISS) and estimated incidence rates in the 20-34 year age group for 2007-2009. We examined whether incidence rates in patients aged 34 and younger could be estimated through the Prescribed Drug Register (PDR) via a proxy for diagnosis of type 1 diabetes; men with at least one and women with at least three prescriptions for insulin were included if they had not been given oral glucose-lowering drugs. We scrutinised the proxy by comparing incidence rates in patients aged 14 and younger with the Swedish Childhood Diabetes Register (SCDR), which has 95-99% ascertainment, and by assessing diabetes type among 18-34 year olds in the National Diabetes Register (NDR).
Results: Incidence rates were two to three times higher than previously reported. The absolute number of cases (2007-2009, age 20-34) was 435 in the DISS, 923 in the NDR, 1,217 in the PDR, 1,431 in all three and 1,617 per the capture-recapture method. Ascertainment in the DISS was ~29% for 2007-2009. The proxy diagnosis in the PDR was highly reliable, while the capture-recapture method presumably generated an overestimate.
Conclusions/interpretation: The incidence of type 1 diabetes in patients aged 34 and younger was two to three times higher than previously reported. The PDR can be used to reliably assess incidence rates in this age group.