Objectives: To evaluate the incidence and relative risk of type 2 diabetes defined by the newly proposed HbA_1c diagnostic criteria in groups categorised by different baseline HbA_1c levels.

Research Design and Methods: Using data from the EPIC-Norfolk cohort with repeat HbA_1c measurements, we estimated the prevalence of known and previously-undiagnosed diabetes at baseline (baseline HbA_1c ≥6.5%) and the incidence of diabetes over 3 years. We also examined the incidence and corresponding odds ratios (OR) by different levels of baseline HbA_1c. Incident diabetes was defined clinically (self-report at follow-up, prescribed diabetes medication or inclusion on a diabetes register) and/or biochemically (HbA_1c ≥6.5% at the second health assessment).

Results: Overall prevalence of diabetes was 4.7%; 41% of prevalent cases were previously undiagnosed. Among 5,735 participants without diabetes at baseline (identified clinically and/or using HbA_1c criteria), 72 developed diabetes over 3 years (1.3%; 95%CI 1.0-1.5), of which half (49%) were identified using the HbA_1c criteria. Six percent of the total population had a baseline HbA_1c in the range 6.0-6.4%; one-third of incident cases arose in this group. Incidence of diabetes in this group was 15 times higher than in those with a baseline HbA_1c of <5.0% (OR 15.5; 95%CI 7.2-33.3).

Conclusions: The cumulative incidence of diabetes defined using a newly proposed HbA_1c threshold in this middle-aged British cohort was 1.3% over 3 years. Targeting interventions to individuals with an HbA_1c of 6.0-6.4% might represent a feasible preventive strategy, although complementary population-based preventive strategies are also needed to reduce the growing burden of diabetes.