What is the role of the Oral Glucose Tolerance Test (OGTT) in the diagnosis of Type 2 Diabetes Mellitus (T2DM)?

From the Guidelines

The oral glucose tolerance test (OGTT) is a recommended diagnostic tool for type 2 diabetes, particularly useful for diagnosing diabetes in individuals with borderline fasting glucose levels or when there's suspicion of diabetes despite normal fasting glucose, as stated in the 2023 standards of care in diabetes 1. To perform this test, the patient fasts overnight (at least 8 hours), then has a fasting blood glucose measurement taken. Next, the patient drinks a solution containing 75 grams of glucose. Blood glucose levels are then measured again at 2 hours after drinking the solution.

• A 2-hour blood glucose level of 200 mg/dL (11.1 mmol/L) or higher indicates diabetes
• Values between 140-199 mg/dL (7.8-11.0 mmol/L) indicate prediabetes or impaired glucose tolerance. The test should be performed in the morning and patients should maintain normal carbohydrate intake for three days before the test. Patients should remain seated during the test and refrain from smoking. The OGTT works by challenging the body's ability to metabolize glucose, revealing impaired insulin secretion or insulin resistance that might not be apparent in a fasting state. While other diagnostic methods exist (fasting glucose, HbA1c), the OGTT remains a valuable test for early detection of glucose metabolism disorders, as supported by the most recent guidelines 1.

From the Research

Oral Glucose Tolerance Test for Diagnosis of Type 2 Diabetes

• The oral glucose tolerance test (OGTT) is a widely used method for diagnosing type 2 diabetes, with a 2-hour plasma glucose (2hPG) value of ≥11.1 mmol/L considered the gold standard for diagnosis 2, 3, 4.
• Recent studies have suggested that a 1-hour plasma glucose (1hPG) value during the OGTT may be a useful predictor of type 2 diabetes, with a cutoff value of ≥11.6 mmol/L having a sensitivity of 0.92 and specificity of 0.91 for detecting type 2 diabetes 3.
• The use of a 1-hour OGTT has been proposed as a potential replacement for the standard 2-hour test, with the advantage of being less time-consuming and potentially more acceptable in clinical settings 2, 5.
• However, the diagnostic accuracy of the OGTT can be influenced by various factors, including fasting plasma glucose (FPG) levels, HbA1c levels, and insulin resistance, highlighting the importance of considering multiple criteria for diagnosis 6, 4.

Diagnostic Criteria and Cutoff Values

• The American Diabetes Association recommends the use of FPG, 2hPG, and HbA1c levels for diagnosing type 2 diabetes, with specific cutoff values for each criterion 6, 4.
• Studies have shown that the combination of FPG and 1hPG can be used to exclude type 2 diabetes in a significant proportion of patients, with a cutoff value of FPG <6.5 mmol/L and 1hPG <12 mmol/L having a high negative predictive value 2.
• The optimal cutoff value for 1hPG has been reported to be around 11.6 mmol/L, although this may vary depending on the population being studied and the specific diagnostic criteria used 3.

Clinical Applications and Future Directions

• The OGTT remains a valuable tool for diagnosing type 2 diabetes, with ongoing research aimed at optimizing its use and improving diagnostic accuracy 6, 5.
• The development of new diagnostic algorithms and cutoff values, such as the use of 1hPG, may help to improve the efficiency and effectiveness of diabetes diagnosis in clinical settings 2, 3.
• Further studies are needed to fully evaluate the potential benefits and limitations of the OGTT and its various diagnostic criteria, including the use of machine learning and other advanced analytical techniques 6, 5.