We followed a pragmatic, high sensitivity approach for case ascertainment with the aim of identifying a) all potential incident diabetes cases and b) excluding all individuals with prevalent diabetes.
Prevalent diabetes was identified on the basis of baseline self-report of a history of diabetes, doctor diagnosed diabetes, diabetes drug use, or evidence of diabetes after baseline with a date of diagnosis earlier than the baseline recruitment date. All ascertained cases with any evidence of diabetes at baseline were excluded.
Ascertainment of incident type 2 diabetes involved a review of the existing EPIC datasets at each centre using multiple sources of evidence including self-report, linkage to primary care registers, secondary care registers, medication use (drug registers), hospital admissions and mortality data (online appendix; table ST1). Information from any follow-up visit or external evidence with a date later than the baseline visit was used. Cases in Denmark and Sweden were not ascertained by self-report, but identified via local and national diabetes and pharmaceutical registers and hence all ascertained cases were considered to be verified (online appendix; table ST1).
To increase the specificity of the case definition for centres other than those from Denmark and Sweden, we sought further evidence for all cases with information on incident type 2 diabetes from fewer than 2 independent sources at a minimum, including individual medical records review in some centres. Follow-up was censored at the date of diagnosis, the 31st of December 2007 or the date of death, whichever occurred first. In total, 12,403 verified incident cases were identified; there were 471 cases in the first year of follow-up and 587 in the second year. Sample size calculations are included in the online appendix (online appendix; figure SF1).