The most common glucose tolerance test is the oral glucose tolerance test (OGTT).
Before the test begins, when you have had nothing to eat or drink for at least 8 hours, a sample of blood will be taken.
You will then be asked to drink a liquid containing a certain amount of glucose (usually 75 grams). Your blood will be taken again every 30 to 60 minutes after you drink the solution.
The test may take up to 3 hours.
A similar test is the intravenous (IV) glucose tolerance test (IGTT). It is rarely used, and is never used to diagnose diabetes. In one version of the IGTT, glucose is injected into your vein for 3 minutes. Blood insulin levels are measured before the injection, and again at 1 and 3 minutes after the injection. The timing may vary. This IGTT is almost always used for research purposes only.
A similar test is used in the diagnosis of growth hormone excess (
acromegaly
) when both glucose and growth hormone are measured after the glucose drink is consumed.