OBJECTIVE: To evaluate A1C for screening and diagnosis of undiagnosed type 2 diabetes defined by oral glucose tolerance testing in clinical and general populations. RESEARCH DESIGN AND METHODS: A1C cut offs (<or =5.5 to rule out diabetes; > or =7.0 to rule in diabetes) were derived from a clinical group (Melbourne Pathology [MP] group: n = 2,494; undiagnosed diabetes 34.6 ) and then evaluated in a population-based sample (AusDiab group: n = 6,015; undiagnosed diabetes 4.6 ). RESULTS: For diabetes in the MP and AusDiab groups, A1C at 5.5 gave sensitivities of 98.7 and 83.5 , while A1C at 7.0 gave specificities of 98.2 and 100 , respectively. Many (61.9-69.3 ) with impaired A1C (5.6-6.9 ) in both populations had abnormal glucose status. CONCLUSIONS: A1C <or =5.5 and > or =7.0 predicts absence or presence of type 2 diabetes, respectively, while at A1C 6.5-6.9 diabetes is highly probable in clinical and population settings. A high proportion of people with impaired A1C have abnormal glucose status requiring follow-up.