Classification of type 1 diabetes and type 2 diabetes based on administrative registry data: a nationwide study from Norway

Abstract Background Type 1 diabetes (T1D) and type 2 diabetes (T2D) differ in health care needs, and precise prevalence estimates by diabetes type are important for health care planning. Data and Methods Using Norwegian health registry data on diagnoses from primary and specialist care and prescribed drugs, we developed a classification method to define T1D and T2D. The Norwegian Diabetes Register for Adults (NDR-A) served as reference standard. Results First, for individuals in the NDR-A, we established a health registry data classification method with three key steps: (A) classifying T1D based on the use of insulin and diagnoses from specialist healthcare, (B) classifying T2D based on diagnoses from both specialist and primary healthcare, and (C) identifying individuals as having T1D if they were diagnosed in primary healthcare and used insulin, but never had a T2D diagnosis. Overall correspondence between NDR-A diagnosis and the classification method was 94.7% for T1D and 97.9% for T2D. For T1D, the correspondence was 97% or higher in age-groups 18-39 and 40-59, 87.6% for those aged 60-79 and 69.4% for individuals older than 80 years. For T2D, the correspondence was 97% or higher for age-groups 40 and above, and 92.1% for those aged 18-39. Applying the classification method to total population data showed that, in 2022, among 4,315,563 individuals aged 18 years and older living in Norway, 31,264 were identified as having T1D (0.72%) and 239,120 were identified as havingT2D (5.5%). Conclusion While classifying diabetes types can be challenging, our registry-based classification corresponded well with the NDR-A.