The Oral Glucose Tolerance Test (OGTT) is a diagnostic tool used to assess how the body processes glucose. It can be used to diagnose gestational diabetes in pregnant women, type 2 diabetes, and prediabetes.
Procedure:
- Fasting: The individual is required to fast overnight for at least 8 hours, but no more than 14 hours.
- Baseline Blood Sample: After fasting, a baseline blood sample is taken to measure the fasting blood glucose level.
- Glucose Intake: The individual then drinks a sugary solution that contains 75 grams of glucose (for adults). For children, the dose is adjusted according to their weight, usually 1.75 grams of glucose per kilogram of body weight (up to a maximum of 75 grams).
- 2 hour Blood Sample: The blood glucose level is repeated 2 hours after consuming the glucose solution.
Interpretation:
Using the Australian Diabetes Society and the World Health Organization’s criteria, the results are interpreted as follows:
Fasting Blood Glucose Level:
- Normal: Less than 5.5 mmol/L
- Impaired Fasting Glucose (Prediabetes): 5.5 to 6.9 mmol/L
- Diabetes: 7.0 mmol/L or higher
2-hour Blood Glucose Level After Drinking the Glucose Solution:
- Normal: Less than 7.8 mmol/L
- Impaired Glucose Tolerance (Prediabetes): 7.8 to 11.0 mmol/L
- Diabetes: 11.1 mmol/L or higher
Pregnancy
For Gestational Diabetes Mellitus (GDM) The International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria, which have been adopted by many Australian institutions, set the following cut-offs for GDM:
- Fasting: ≥5.1 mmol/L (cf ≥5.5)
- 1 Hour: ≥10.0 mmol/L after a 75g glucose load
- 2 Hours: ≥8.5 mmol/L after a 75g glucose load (cf ≥11.1)
Test is usually done at 24-28 weeks