Oral Glucose Tolerance Test
Impaired Glucose Tolerance (Pre-diabetes)
What is Impaired Glucose Tolerance (IGT)?
Impaired glucose tolerance is a condition preceding the development of type 2 diabetes. It occurs when the blood glucose level after a standard glucose drink is higher than normal, but not high enough to be classified as diabetes. In this condition, insulin production is decreased or insulin is less effective.
How is IGT diagnosed?
This condition is diagnosed by an oral glucose tolerance test (OGTT), the same test that is used to detect diabetes. This is performed by taking a fasting blood test to check for blood glucose level. Then the person will be given a glucose drink consisting of 75g glucose. A second blood test for blood glucose is repeated 2 hours after taking the glucose drink. People with fasting blood glucose less than 7 mmol/L), but abnormal 2-hour blood glucose (7.8-11.1mmo./L) have IGT. Subjects with IGT have increased risk of heart disease, stroke and diabetes.
If the fasting blood glucose equals or is higher than 7 mmol/L and/or 2-hour blood glucose is higher than 11.1 mmo/L, then type 2 diabetes is diagnosed.
Subjects who have fasting blood glucose between 5.5 and 7 mmol/l is known to have impaired fasting glucose. These subjects are at also risk of developing diabetes.
In 1997, the American Diabetes Association (ADA) expert panel recommended that doctors use a single fasting blood glucose test to screen for diabetes. This is because the test is easier and less costly than OGTT. While a single fasting glucose test is useful in clinical setting to detect diabetes, OGTT is more sensitive in identifying people with a glucose problem that may first appear only after a glucose challenge. In addition, OGTT has the advantage in detecting the pre-diabetes state - impaired glucose tolerance.
Why is IGT important?
Impaired glucose tolerance is associated with increased risk of heart disease and stroke and is an early indicator for development of type 2 diabetes. Once a person develops type 2 diabetes, he/she will have further increased risk of diabetic vascular complications affecting eyes, kidneys, nerves, feet, etc. Very often, IGT occurs as part of the metabolic syndrome which includes high blood pressure, abnormal blood fat level, high urate (causing gout) and obesity. All these risk factors contribute to increased risk for cardiovascular disease. Hence identifying such individuals help early detection of these risk factors which can be treated early to reduce their risk of having coronary heart disease and stroke.
Who is at risk of having IGT?
The following are risk factors for developing IGT
- Obesity
- Family history of diabetes
- Age > 45
- Gestational diabetes (diabetes during pregnancy)
- Hypertension
- Hypercholesterolaemia(especially, high triglycerides, low HDL-C which is a good cholesterol)