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Background Elevated plasma glucose levels on admission (APG) are very common in patients with acute coronary syndrome (ACS) and can be the first indication of diabetes mellitus. Objective To provide insight into the prevalence of previously undiagnosed diabetes and to compare different methods of diagnosing diabetes in patients with ACS. Methods Patients with ACS with elevated APG who participated in the BIOMArCS 2 glucose trial underwent an oral glucose tolerance test (OGTT) prior to discharge. 130 patients were included who underwent metabolic assessment. Of these, 109 had an OGTT and 13 patients had pre-existing diabetes. Results The OGTT results were categorised as (previously) undiagnosed diabetes in 35% of patients (fasting plasma glucose (FPG) ≥7.0 mmol/l or 2-h post-load glucose ≥11.1 mmol/l) and impaired glucose metabolism in 44% (FPG 6.1–6.9 mmol/l or post-load glucose 7.8–11.0 mmol/l), so only 21% had a normal glucose metabolism. Undiagnosed diabetes could not be adequately predicted with APG, FPG or HbA1c (area under the ROC curve 0.61, 0.75 and 0.72, respectively). Patients with abnormal glucose metabolism were significantly older, had higher admission HbA1c values, a higher Killip classification and more often had a prior stroke than patients with normal glucose metabolism. Conclusion 79% of hyperglycaemic patients with ACS were found to have abnormal glucose metabolism. As APG, HbA1c and FPG had a low sensitivity to detect undiagnosed diabetes, an OGTT appears to be the best test to assess the presence of previously undiagnosed diabetes or impaired glucose metabolism in hyperglycaemic patients with ACS.