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The diagnostic criteria for diabetes and prediabetes have changed over time. They are currently based on the results of 3 blood tests—the fasting blood glucose test, the oral glucose tolerance test, and the hemoglobin A1c test. All of these tests identify people with diabetes or at risk of developing diabetes. However, the number of people that these tests identify and their characteristics vary between the 3 tests. For example, A1c varies by race/ethnicity independent of blood glucose levels. The implications of demographic differences in estimates of undiagnosed diabetes and prediabetes when using different laboratory tests are not known. Research is ongoing to ascertain the best use of laboratory blood tests to detect people who may have diabetes or prediabetes and to improve the understanding of who has these conditions (