What is the purpose and procedure of the Oral Glucose Tolerance Test (OGTT)?

Oral Glucose Tolerance Test (OGTT): Purpose and Procedure

The Oral Glucose Tolerance Test (OGTT) is primarily used to diagnose diabetes mellitus, impaired glucose tolerance (IGT), and impaired fasting glucose (IFG) when fasting plasma glucose and HbA1c results are inconclusive. 1

Purpose of OGTT

• OGTT is particularly valuable for detecting impaired glucose tolerance (IGT), which is a significant risk factor for future diabetes and cardiovascular disease 1
• It helps identify patients with isolated post-challenge hyperglycemia who would be missed by fasting glucose tests alone 2
• The American Diabetes Association recommends OGTT as one of three diagnostic criteria for diabetes, alongside fasting plasma glucose and HbA1c 3
• OGTT is especially useful in diagnosing gestational diabetes mellitus (GDM) in pregnant women 3

Diagnostic Criteria

• Diabetes is diagnosed if 2-hour plasma glucose is ≥200 mg/dL (11.1 mmol/L) during an OGTT 3, 1
• Impaired glucose tolerance (IGT) is diagnosed if 2-hour plasma glucose is between 140-199 mg/dL (7.8-11.0 mmol/L) 1
• Impaired fasting glucose (IFG) is diagnosed if fasting plasma glucose is between 100-125 mg/dL (5.6-6.9 mmol/L) according to ADA criteria 1

OGTT Procedure

• The patient should fast for at least 8 hours prior to the test 1, 3
• A baseline fasting plasma glucose sample is collected 1
• The test involves administering a glucose load containing the equivalent of 75g anhydrous glucose dissolved in water 3, 1
• For standard diabetes diagnosis, blood samples are collected at fasting and 2 hours after glucose load 1
• For gestational diabetes, different protocols may be used:
  • One-step approach: Perform a diagnostic OGTT directly 3
  • Two-step approach: Initial screening with a 50-g glucose challenge test (GCT), followed by a 100-g OGTT for those exceeding threshold values 3
• The test should be performed in the morning after an overnight fast of 8-14 hours 3
• The patient should remain seated and not smoke throughout the test 3

Clinical Considerations

• In the absence of unequivocal hyperglycemia, abnormal results should be confirmed by repeat testing on a different day 3
• The American Diabetes Association recommends repeat testing at least at 3-year intervals if tests are normal 1
• Certain medications, including glucocorticoids and nicotinic acid, may affect test results by producing hyperglycemia 1
• While the ADA has suggested that OGTT is not recommended for routine clinical use, it remains valuable in specific scenarios where fasting glucose or HbA1c may be inconclusive 3, 4

Special Populations

• For pregnant women, GDM screening should be performed between 24-28 weeks of gestation for average-risk women 3
• High-risk pregnant women should be tested as soon as possible during pregnancy 3
• For children and adolescents at risk for type 2 diabetes, testing should be performed every 3 years in overweight youth (BMI >85th percentile) with additional risk factors 3

Advantages and Limitations

• OGTT is more sensitive for the diagnosis of diabetes and pre-diabetes than fasting plasma glucose alone, but is more time-consuming and expensive as a screening procedure 3, 2
• Without OGTT, a significant percentage of diabetic patients (up to 25.8%) and IGT subjects (up to 78.4%) may remain unidentified 2
• Recent research suggests that 1-hour plasma glucose measurements during OGTT may provide valuable diagnostic information and could potentially replace the conventional 2-hour test 4

The OGTT remains a cornerstone diagnostic tool for diabetes and prediabetes, providing crucial information about glucose metabolism that cannot be obtained through fasting glucose or HbA1c measurements alone.