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The oral glucose tolerance test (OGTT) is recommended to screen for diabetes in patients with coronary artery disease. We hypothesized that testing for glycated hemoglobin (HbA1c), in addition to the OGTT, in screening for abnormal glucose regulation may help to reveal patients with β-cell function impairment.Patients with no history of diabetes who were admitted for coronary angiography were recruited to undergo an OGTT and HbA1c test 2–4 weeks after hospital discharge. β-cell function and insulin resistance were assessed using the homeostasis model assessment (HOMA-β and HOMA-IR, respectively). For patients with normal glucose tolerance (NGT) based on the OGTT, we compared HOMA-β between two subgroups of patients using an HbA1c cutoff of 39 mmol/mol or 42 mmol/mol. For patients with prediabetes based on an OGTT, we compared the HOMA-β between two subgroups of patients using an HbA1c cutoff of 48 mmol/mol.A total of 1044 patients were analyzed. In patients with NGT by OGTT (n=432), those with an HbA1c ≥42 mmol/mol had a lower HOMA-β compared to those with an HbA1c <42 mmol/mol (107±82 vs. 132±96, p=0.018). In patients with prediabetes by OGTT (n=423), those with an HbA1c ≥48 mmol/mol had a lower HOMA-β compared to those with an HbA1c <48 mmol/mol (91±52 vs. 120±88, p=0.003). No significant between-group difference in HOMA-IR was noted.The use of HbA1c in addition to the OGTT in screening for abnormal glucose regulation helped to reveal patients with early β-cell function impairment.