The 1-Hour Glucose Tolerance Test Not the Fasting Glucose Testing Accurately Identifies Diabetes RiskMost doctors who screen you for diabetes give you a fasting glucose test or an A1c test--though the American Diabetes Association specifically states that the A1c test should not be used for diagnosing diabetes. But research published in 2008 that was based on studying a group of 2,442 subjects who were free of type 2 diabetes at the beginning of the study found that fasting glucose tests were a very poor predictor of who in this group would develop diabetes.
The researchers found that people whose one hour glucose tolerance test results were over 155 mg/dl and who had markers for metabolic syndrome--such as a concentration of fat around the belly and high blood pressure were those who were more accurately predicted to be likely to develop diabetes.
Fasting Versus Postload Plasma Glucose Concentration and the Risk for Future Type 2 Diabetes Muhammad A. Abdul-Ghani et al. Diabetes Care 32:281-286, 2009 DOI: 10.2337/dc08-1264
the above is from Jenny Ruhl's site