HbA1c predicts the likelihood of having impaired glucose tolerance in high-risk patients with normal fasting plasma glucose

Background: Although the oral glucose tolerance test (OGTT) is the 'gold standard' for diagnosing prediabetes/diabetes, it is inconvenient for the patient and time consuming. The only alternative simple screening test is fasting plasma glucose (FPG). FPG concentrations of > 6.0 mmol/L represent prediabetes/diabetes. FPG concentrations of <= 6.0 mmol/L may be considered 'normal', although some such patients will demonstrate abnormal glucose tolerance when subjected to an OGTT. We have evaluated the use of glycated haemoglobin (HbA(1c)) as a screening test for diabetes or impaired glucose tolerance (IGT) in patients who have risk factors for diabetes but FPG <= 6.0 mmol/L. Methods and results: A total of 580 patients with at least two risk factors for diabetes underwent an OGTT and HbA(1c) measurement. In all, 225 patients had a FPG <= 6.0 mmol/L and met the inclusion criteria. Of these, 23.1% (n = 52) had an abnormal OGTT result (45 had IGT and 7 had diabetes). Subjects with abnormal glucose tolerance had a higher percentage of HbA(1c) than subjects with normal glucose tolerance (P < 0.001). An HbA(1c), of 5.6% gave an optimal sensitivity of 72% and specificity of 77% to predict a 2 h plasma glucose >= 7.8 mmol/L. Conclusion: The use of FPG concentration followed by selective measurement of HbA(1c) in patients who are at high risk of developing diabetes may represent a reasonable approach to identifying patients requiring an OGTT.