Diagnostic Performance of HbA1c for Detecting OGTT-Diagnosed Diabetes in Obese Individuals with Suspected Prediabetes

Background: We aimed to investigate the diagnostic performance between the oral glucose tolerance test (OGTT) and HbA1c in diagnosing prediabetes and diabetes among obese individuals, and to evaluate the diagnostic performance of HbA1c for detecting OGTT-defined diabetes in obese individuals referred for evaluation of suspected prediabetes. Methods: Individuals with prediabetes were included between 1 January 2020 and 31 December 2022. Participants were categorized as mildly, moderately, morbidly, or super obese based on body mass index (BMI). According to the 75 g OGTT results, patients were classified into three groups: isolated impaired fasting glucose (IFG), combined IFG + impaired glucose tolerance (IGT), and overt type 2 diabetes mellitus (T2DM). The threshold HbA1c value for T2DM diagnosis in obese patients was determined based on OGTT outcomes. Results: Of the 139 prediabetic obese patients included, 115 (82.7%) were female, with a mean age of 45.18 ± 11.74 years. Based on BMI, 34 patients (24.5%) were mildly obese, 41 (29.5%) moderately obese, 49 (35.3%) morbidly obese, and 15 (10.8%) super obese. According to the 75 g OGTT results, 37.4% (n = 52) had isolated IFG, 45.3% (n = 63) had combined IFG + IGT, and 17.3% (n = 24) had overt T2DM. A weak-moderate positive correlation was observed between HbA1c and fasting blood glucose (Spearman's rho = 0.263, p = 0.002). ROC-AUC analysis showed that HbA1c had significant discriminatory power in detecting T2DM diagnosed by the 75 g OGTT (AUC = 0.881, 95% CI: 0.816-0.946, p < 0.001). The optimal HbA1c cut-off was 6.15%, with 83.3% sensitivity and 80% specificity. The positive predictive value was 46.1%, and the negative predictive value was 95.8%. Conclusions: An HbA1c threshold of 6.15% demonstrated optimal performance for detecting OGTT-defined diabetes in obese individuals with suspected prediabetes. This value should not be interpreted as a population-wide diagnostic threshold. These findings indicate that HbA1c may serve as a useful screening tool to identify obese individuals who warrant confirmatory OGTT testing, rather than as a stand-alone diagnostic criterion. Further large-scale studies are warranted to confirm these results and support future clinical guidelines.