Oral Glucose Tolerance Test—1-Hour and 2-Hour Plasma Glucose, Curve Shape, and Diagnosis—A Retrospective Study

Introduction and Objective: Recent recommendations emphasize that the 1-hour plasma glucose level obtained from the oral glucose tolerance test (OGTT) is a crucial method for diagnosing diabetes and assessing dysglycemia. The objective of this study is to compare the diagnostic performance of both 1-hour plasma glucose (1-h PG) and 2-hour plasma glucose (2-h PG), while also examining the common glucose curve shapes within the studied population. Methods: Conducted in a Brazilian laboratory, this retrospective quantitative cross-sectional study collected data from patients who underwent OGTT with 3 to 7 time points during the period from 2021 to 2024. Participants were individuals aged 18 years and older of both sexes, excluding pregnant women and those on antidiabetic medications. Tests with five or more time points were analyzed for curve shape. Statistical analyses included interrater reliability assessments and chi-square tests, with significance set at p < 0.05. Results: Data from 1,805 patients revealed diagnoses based on 1-h PG levels: normal glucose tolerance (NGT) at 62.7%, impaired glucose tolerance (IGT) at 29.8%, and diabetes at 7.5%. Diagnoses for 2-h PG were 76.3% NGT, 18.1% IGT, and 5.6% diabetes. Moderate agreement between the two tests was observed (Kappa = 0.480; p < 0.001). Notably, a total of 283 patients (15.7%) exhibited NGT at 2-h PG but IGT at 1-h PG, while 20 patients (1.1%) had NGT at 2-h PG and diabetes at 1-h PG. Among 1,358 analyzed curves, biphasic curves were present in 89.7% of NGT patients, whilst 53.8% of IGT or diabetic patients had monophasic curves (p < 0.001). The monophasic shape correlated with 85.5% of IGT and 89.1% of diabetic patients (p < 0.001). Conclusion: Findings indicate that 1-h PG parameters show higher sensitivity for diagnosing IGT compared to 2-h PG parameters. Additionally, biphasic glucose responses are prevalent in NGT patients, while monophasic responses are more common in individuals with IGT and diabetes. Disclosure: J.F. Castro: None. D.B. Macedo: None. R.A. Magalhaes: None. C.M. Figueirêdo: None. R. Petrola: None. M.G. Teles: None.