From the Guidelines
The 75-gram oral glucose tolerance test (OGTT) protocol for diagnosing type 2 diabetes involves measuring blood glucose levels before and after consuming a standardized glucose solution, with a 2-hour plasma glucose level ≥200 mg/dL (11.1 mmol/L) indicating diabetes, as recommended by the most recent and highest quality study 1. The patient should fast for at least 8 hours (typically overnight) before the test. On arrival, a baseline fasting blood glucose sample is taken. The patient then drinks a solution containing exactly 75 grams of anhydrous glucose dissolved in water (usually 300 mL) within 5 minutes. Some key points to consider when performing the test include:
- Blood samples are collected at specific intervals, most importantly at the 2-hour mark after glucose consumption.
- A value between 140-199 mg/dL (7.8-11.0 mmol/L) indicates impaired glucose tolerance.
- The test should be performed in the morning, and patients should maintain normal carbohydrate intake for three days before testing while avoiding smoking, caffeine, and exercise during the test.
- Certain medications that affect glucose metabolism should be temporarily discontinued if possible, as noted in 1. This test works by challenging the body's ability to metabolize glucose; in diabetes, insulin production or effectiveness is impaired, resulting in prolonged elevation of blood glucose levels after the glucose challenge, which is supported by earlier studies such as 1 and 1.
From the Research
Oral Glucose Tolerance Test Protocol
The oral glucose tolerance test (OGTT) is a widely used method for diagnosing type 2 diabetes mellitus (T2DM) and prediabetes. The test involves administering a glucose solution orally and measuring blood glucose levels at various time points.
Procedure
- The patient is asked to fast for at least 8 hours before the test
- A baseline blood sample is taken to measure fasting blood glucose levels
- The patient is then given a glucose solution containing 75 grams of glucose to drink
- Blood samples are taken at 30 minutes, 1 hour, 2 hours, and sometimes 3 hours after glucose ingestion to measure blood glucose levels 2
Interpretation of Results
- The results of the OGTT are interpreted based on the blood glucose levels at the different time points
- A 2-hour plasma glucose level of 200 mg/dl or higher is indicative of diabetes, while a level of 140-199 mg/dl is indicative of impaired glucose tolerance (prediabetes) 2
- Some studies suggest that measuring the 1-hour plasma glucose level may also be useful in identifying high-risk individuals 2
Relation to Type 2 Diabetes Treatment
- The OGTT is not only used for diagnosis but also for monitoring the effectiveness of treatment in patients with T2DM
- Studies have shown that certain medications, such as metformin, can improve glucose tolerance and reduce blood glucose levels in patients with T2DM 3, 4
- Other medications, such as sodium-glucose co-transporter 2 inhibitors, have also been shown to be effective in improving glycemic control and reducing the risk of hypoglycemia in patients with T2DM 5