What is the purpose of an Oral Glucose Tolerance Test (OGTT) for diagnosing Diabetes Mellitus (DM)?

From the Guidelines

The oral glucose tolerance test (OGTT) is the recommended diagnostic test for diabetes, particularly for gestational diabetes in pregnant women, and should be performed with a 75-g glucose load after an overnight fast of at least 8 hours, with plasma glucose measurements at fasting, 1 hour, and 2 hours. When performing the OGTT, it is essential to follow the standardized protocol to ensure accurate results, as outlined in the 2023 standards of care in diabetes 1. The test involves:

• Fasting for at least 8 hours
• Drinking a 75-g glucose solution
• Measuring plasma glucose levels at fasting, 1 hour, and 2 hours The diagnosis of gestational diabetes mellitus (GDM) is made when any of the following plasma glucose values are met or exceeded:
• Fasting: 92 mg/dL (5.1 mmol/L)
• 1 hour: 180 mg/dL (10.0 mmol/L)
• 2 hour: 153 mg/dL (8.5 mmol/L) Alternatively, a two-step strategy can be used, involving a 50-g glucose load test (GLT) followed by a 100-g OGTT if the initial test is abnormal, with diagnosis made when at least two of the following plasma glucose levels are met or exceeded:
• Fasting: 95 mg/dL (5.3 mmol/L)
• 1 hour: 180 mg/dL (10.0 mmol/L)
• 2 hour: 155 mg/dL (8.6 mmol/L)
• 3 hour: 140 mg/dL (7.8 mmol/L) The OGTT is a valuable tool for diagnosing diabetes and assessing glucose tolerance, and its results should be interpreted in the context of clinical guidelines and patient-specific factors, as recommended by the American College of Obstetricians and Gynecologists and outlined in the 2023 standards of care in diabetes 1.

From the Research

Oral Glucose Tolerance Test for Diabetes

• The oral glucose tolerance test (OGTT) has been widely used for diagnosing diabetes for decades 2, 3.
• The test measures the body's ability to regulate blood sugar levels after consuming a sugary drink 4.
• The OGTT has been compared to other screening tests, such as fasting blood glucose, and has been found to be more effective in detecting early diabetes and impaired glucose tolerance (IGT) 2.
• The test typically involves measuring blood glucose levels at 2 hours after consuming the sugary drink, but some studies have suggested that measuring glucose levels at 1 hour or 30 minutes may also be useful 3, 4.

Diagnostic Criteria

• The American Diabetes Association (ADA) has suggested abandoning the OGTT in favor of simpler screening tests, but some studies have argued that the OGTT is still a valuable diagnostic tool 2, 3.
• Different diagnostic criteria have been proposed, including the use of fasting blood glucose, 2-hour post-load glucose, and HbA1c levels 3, 5.
• The OGTT has been found to be useful in identifying individuals with impaired glucose tolerance and those at risk of developing type 2 diabetes 2, 3.

Clinical Applications

• The OGTT has been used in clinical practice to diagnose gestational diabetes mellitus (GDM) and to identify women at risk of developing pre-diabetes and type 2 diabetes after pregnancy 5, 6.
• The test has also been used to assess insulin sensitivity and glucose utilization in research studies 4.
• Some studies have suggested that the OGTT may be useful in identifying individuals with different metabolic entities, such as isolated fasting hyperglycemia or combined hyperglycemia 5.