What is a Glucose Tolerance Test?

A glucose tolerance test is a test doctors use to measure the body's efficiency at metabolizing sugar. When the body metabolizes sugar, it essentially breaks it down to use for energy. If the body does not metabolize sugar as well as it should, diabetes may be the result. The glucose tolerance test can be used to detect type 2 diabetes, which is the most frequently diagnosed type of diabetes. It is also used to diagnose gestational diabetes, which is a type of diabetes that may develop during pregnancy.

Doctors perform a glucose tolerance test to check for problems with the way a person's body handles glucose, which is blood sugar, after the person has consumed a meal. In preparation for the test, patients are typically asked to avoid eating and drinking anything for about eight hours prior to the test, although they should consume their normal diets up until the beginning of the eight-hour fasting period. Often, glucose tolerance tests are scheduled for first thing in the morning, allowing patients to fast overnight while they are sleeping.

Doctors perform the glucose tolerance test in several steps. As the first step, the patient's blood is taken. A medical professional may either use a needle to take blood from a vein or a medical device to prick a finger and take a small amount of blood from there. This blood is then used to evaluate the patient's blood sugar level while fasting.

To test for type 2 diabetes, doctors have the patient drink about a cup (226.79 grams) of glucose solution, which is very syrupy and sweet; in fact, it often tastes similar to a very sweet soda that has lost its bubbles. After drinking the solution, the patient waits about two hours and has his blood tested once more. The solution used to test for type 2 diabetes usually contains about 2.6 ounces (75 grams) of sugar. Patients who are tested for gestational diabetes also drink this solution; however, its sugar content is 3.5 ounces (100 grams), and the patient's blood may be taken one, two, and three hours after it is consumed. Sometimes, pregnant patients only have their blood tested one hour after drinking the glucose solution and do not need further blood testing.

Most people don't find the glucose tolerance test particularly uncomfortable. Some people feel nauseous after drinking the glucose solution, however, and others feel uncomfortable with waiting so long to eat. Additionally, some people note discomfort from the needle or medical device used to draw their blood.