The authors of the article cited above noticed an error in the way they had described their definitions of type 1 and type 2 diabetes in the research design and methods section.
The description as published is as follows:
Diabetes type is classified by the health practitioner completing registration. However, for the current study, type 1 diabetes status was assigned to registrants who were classified as type 1 on the NDSS and were diagnosed before the age of 30 years, and the time between diagnosis date and date of first insulin use was <1 year. For those missing data on date of diagnosis (59.1% type 1 diabetes and 36.1% type 2 diabetes) or insulin initiation date (many of whom registered in the early years of the operation of the NDSS and had had diabetes for a number of years), we classified people as type 1 if they were recorded as type 1 on the registry, were taking insulin, and were registered at ≤45 years of age. We chose 45 years as the cutoff to minimize the number of people with type 1 diabetes that we would miss, without misclassifying significant numbers of people with type 2 as type 1 [Kenny et al., 1995]. All others were classified as type 2 diabetic.
Amended description is as follows:
Diabetes type is classified by the health practitioner completing registration. However, for the current study, type 1 diabetes status was assigned to registrants who were recorded as type 1 on the NDSS registry, were registered at <45 years of age, and were taking insulin. Registration date was used as a proxy for diagnosis date as a large proportion of registrants (59.1% with type 1 diabetes and 36.1% with type 2 diabetes) were missing date of diagnosis, many of whom registered in the early years of the operation of the NDSS and had had diabetes for a number of years. We chose 45 years as the cutoff to minimize the number of people with type 1 diabetes that we would miss, without misclassifying significant numbers of people with type 2 diabetes as type 1 diabetes [Kenny et al., 1995]. In addition, registrants who were recorded as having type 2 diabetes on the registry, were diagnosed before the age of 30 years, and were taking insulin within 1 year of diagnosis date were reclassified as having type 1 diabetes. All others were classified as having type 2 diabetes.
Had the authors analyzed the data according to how the definition reads in the article, they would have excluded approximately 13% of those with type 1 diabetes. These patients were all insulin treated and were all registered on the NDSS before the age of 45 years, and the majority were registered with the NDSS in the early years of its existence and therefore did not have an age at diagnosis available. The authors believe that the most appropriate classification of these patients is type 1 diabetes and that the published results, in which they were classified as type 1 diabetes, are therefore the appropriate ones. Nevertheless, the authors have examined the effect of differential coding by conducting some analyses using both the method that appeared in the research design and methods section and the amended method. These results and comparisons are shown in Supplementary Tables 1a and 2a. Standardized incidence ratios (SIRs) for type 1 and type 2 diabetes in men and women combined are presented in Supplementary Table 1a and in men and women separately are presented in Supplementary Table 2a. The authors observed a magnitude of difference between the two versions of results of less than 10% for type 1 diabetes and even less for type 2 diabetes, with overlapping 95% CIs. Stratified by sex, these differences are larger, probably driven by the small number of some cancers in the population with the type 1 diabetes.
Changing the description of the definition does not require any change to the results.
