3 Hour Glucose Tolerance Test: How Do Doctors Diagnose Diabetes Mellitus?

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About Glucose Tolerance Tests

Glucose is a simple carbohydrate that the body uses as its most basic form of energy. Diabetic patients suffer either from insulin resistance or insulin deficiency depending on the type of diabetes they have. Physicians must perform a test to determine if a patient is diabetic or “pre-diabetic," a condition known as impaired glucose tolerance (IGT). Pregnant mothers often have the test performed between their 24th and 28th weeks of pregnancy to screen for a condition called gestational diabetes. This affects pregnant women during the later stages of pregnancy and, if untreated, can have negative consequences on the child.

Some physicians use the 3 hour glucose tolerance test, although many may order the test in a duration of between one to six hours; it is also known as an oral glucose tolerance test (OGTT) to differentiate it from the intravenous glucose tolerance test (IGTT). The three-hour test is becoming less common, but at one time was the gold standard of diabetes diagnosis.

Preparing for the Test

Patients who have been ordered to undergo a 3 hour glucose tolerance test must prepare for at least three days prior to the test. During these three days, you must eat normal meals, paying special attention that you are eating normal portions of carbohydrates. As the day of the test draws closer, you must not eat anything, or fast, for eight to ten hours prior to the beginning of the test. For this reason, many OGTTs are scheduled in the morning. You will not be able to eat during the test; but you may drink plain water in moderation.

Some patients become light-headed, nauseous or physically ill as a result of the fasting and the sudden changes in blood sugar. This is normal. Just let your doctor or a nurse know, and they will take care of you.

Test Procedure

Blood glucose tests are fairly standardized in how they are performed. Only a few differences exist, specifically if the test in being performed on a pregnant mother or a child. The test for healthy adults is usually performed along a set line of procedures.

A physician, nurse or assistant will draw your blood when you first arrive. This is to determine your fasting blood glucose (FBG), your baseline measurement from which to compare your body’s reaction. If your fasting blood glucose measures below 130 mg/dL, you will be given 75 mL of a glucose solution and ten minutes in which to drink it. The exact time you finish will be noted. Your blood will be drawn after 30 minutes, one hour, two hours and three hours.

Pregnant mothers will receive 100 mL of the glucose solution, as opposed to the 75 for normal adults. Children will receive a dose of the glucose solution based upon their weight.


In healthy adults, blood glucose levels rise sharply after eating and peak between 30 and 60 minutes later. After this, the blood glucose levels steadily return to normal, without dipping too low after another one to two hours. Patients displaying diabetic symptoms will show a higher than normal peak in the same time frame as healthy individuals. However, the return to normal occurs much slower.

After a 3 hour glucose tolerance test, physicians compare the blood glucose levels from all of the blood samples. Fasting blood glucose levels should be 60 to 100 mg/dL. The one-hour sample should be below 200 mg/dL, and the two-hour sample should be lower than 140 mg/dL. Two-hour levels between 140 and 200 mg/dL indicate impaired glucose tolerance, while levels above 200 mg/dL generally indicate diabetes. Patients displaying diabetic results will normally be required to undergo another OGTT.

Pregnant mothers can expect slightly different values. Fasting levels should be below 95 mg/dL, the one-hour level should be below 180 mg/dL and two-hour sample levels less than 155 mg/dL. After three hours, blood glucose should have returned to 140 mg/dL or less.


Medline Plus: Glucose Tolerance Test

WebMD: Glucose Tolerance Test for Gestational and Type 2 Diabetes

Lab Tests Online: Glucose Tests: The Test

University of Connecticut Health Center John Dempsey Hospital Clinical Laboratory Services: Glucose Tolerance Test Specimen Collection

American Pregnancy Association: Glucose Challenge Screening & Glucose Tolerance Test