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Several studies have suggested that HbA1c levels may predict incident diabetes. With new recommendations for use of HbA1c in diagnosing diabetes, many patients with HbA1c results below the diagnostic threshold will be identified. Clinicians will need to categorize risk for a subsequent diabetic diagnosis in such patients. The objective of this study was to determine the ability of HbA1c to predict the incidence of a diabetic diagnosis.We performed a historical cohort study using electronic medical record data from two Department of Veterans Affairs Medical Centers. Patients (n = 12,589) were identified with a baseline HbA1c <6.5% between January 2000 and December 2001 and without a diagnosis of diabetes. Patients (12,375) had at least one subsequent follow-up visit. These patients were tracked for 8 years for a subsequent diagnosis of diabetes.During an average follow-up of 4.4 years, 3,329 (26.9%) developed diabetes. HbA1c ≥5.0% carried a significant risk for developing diabetes during follow-up. When compared with the reference group (HbA1c <4.5%), HbA1c increments of 0.5% between 5.0 and 6.4% had adjusted odds ratios of 1.70 (5.0–5.4%), 4.87 (5.5–5.9%), and 16.06 (6.0–6.4%) (P < 0.0001). Estimates of hazard ratios similarly showed significant increases for HbA1c ≥5.0%. A risk model for incident diabetes within 5 years was developed and validated using HbA1c, age, BMI, and systolic blood pressure.The incidence of diabetes progressively and significantly increased among patients with an HbA1c ≥5.0%, with substantially expanded risk for those with HbA1c 6.0–6.4%.