Comparison of HbA1c and oral glucose tolerance test for diagnosis of diabetes in patients with coronary artery disease

Measurement of glycated hemoglobin A1c (HbA1c) to diagnose diabetes mellitus (DM-2) is recommended by several expert groups. DM-2 occurs very frequently among patients with coronary artery disease (CAD). Therefore, we aimed to investigate the diagnostic strengths of HbA1c and oral glucose tolerance test (OGTT) in detecting latent glucometabolic disturbances among patients with CAD. One hundred ninety-nine consecutive patients admitted with CAD were included in this observational study. Fasting plasma glucose as well as HbA1c measurement was performed in all study participants and those without preexisting DM-2 underwent an OGTT. Patients were subdivided according to their medical history into those with previous DM-2 (n = 37). The remaining 162 patients underwent OGTT, which revealed 39 patients with diabetes (DM-(OGTT)), 35 with impaired glucose tolerance (IGT), 20 with impaired fasting glucose (IFG) and 68 with normal glucose tolerance (NGT). Using HbA1c resulted in 6.8% DM and 45.6% at risk (HbA1c 5.7-6.4%) diagnosis. OGTT identified 24.1% DM (p = 0.002 compared with HbA1c) and 21.6% IGT patients. Among those with intermediate HbA1c (5.7-6.4%) 26.5% patients were NGT and only 30.9% displayed DM-2 by use of OGTT. Among patients with HbA1c of < 5.7%, 44% (n = 31) of patients had disturbed glucose metabolism. Using receiver-operating curve HbA1c cutoff with the highest sensitivity and specificity was found to be 5.8%. There is a large discordance between OGTT and HbA1c in terms of detecting latent DM-2 in patients with CAD. Measurement of HbA1c could result in lower propensity of DM-2 diagnosis.