Elevated HbA(1c) and Fasting Plasma Glucose in Predicting Diabetes Incidence Among Older Adults: Are two better than one?

OBJECTIVE: To determine which measures—impaired fasting glucose (IFG), elevated HbA(1c), or both—best predict incident diabetes in older adults. RESEARCH DESIGN AND METHODS: From the Health, Aging, and Body Composition study, we selected individuals without diabetes, and we defined IFG (100–125 mg/dL) and elevated HbA(1c) (5.7–6.4%) per American Diabetes Association guidelines. Incident diabetes was based on self-report, use of antihyperglycemic medicines, or HbA(1c) ≥6.5% during 7 years of follow-up. Logistic regression analyses were adjusted for age, sex, race, site, BMI, smoking, blood pressure, and physical activity. Discrimination and calibration were assessed for models with IFG and with both IFG and elevated HbA(1c). RESULTS: Among 1,690 adults (mean age 76.5, 46% men, 32% black), 183 (10.8%) developed diabetes over 7 years. Adjusted odds ratios of diabetes were 6.2 (95% CI 4.4–8.8) in those with IFG (versus those with fasting plasma glucose [FPG] <100 mg/dL) and 11.3 (7.8–16.4) in those with elevated HbA(1c) (versus those with HbA(1c) <5.7%). When FPG and HbA(1c) were considered together, odds ratios were 3.5 (1.9–6.3) in those with IFG only, 8.0 (4.8–13.2) in those with elevated HbA(1c) only, and 26.2 (16.3–42.1) in those with both IFG and elevated HbA(1c) (versus those with normal FPG and HbA(1c)). Addition of elevated HbA(1c) to the model with IFG resulted in improved discrimination and calibration. CONCLUSIONS: Older adults with both IFG and elevated HbA(1c) have a substantially increased odds of developing diabetes over 7 years. Combined screening with FPG and HbA(1c) may identify older adults at very high risk for diabetes.