Glucose Tolerance Test

At the beginning of the third trimester between 26-28 weeks you will need to do the one hour glucose tolerance test. This is a test to see if you have developed gestational diabetes. This test may be unpleasant but it is very important. If gestational diabetes is identified and treated you and the baby will be fine, if it is not identified and treated, major problems can develop. The one hour glucose tolerance test is also called the one hour GTT or the O’Sullivan test.

There are different ways the one hour glucose tolerance test can be done. In my practice I ask the patient to be fasting (nothing to eat or drink after going to bed) and have the test done first thing in the morning. A fasting blood sugar is measured by drawing blood. The patient is then asked to drink a syrupy sweet liquid that contains 50 grams of glucose. Blood is drawn one hour later. This allows us to see how the body handles a known amount of sugar to see if it is processing it correctly. An abnormal fasting value is enough to diagnose gestational diabetes. If the one hour value is high then a second test, called the three hour glucose tolerance test, is usually ordered.

The three hour glucose tolerance test is fairly unpleasant. Like the one hour GTT, a fasting blood test is done. Then the patient drinks twice as much of the sweet stuff and blood tests are done each hour for three hours. If the fasting blood test is high, or if the fasting blood test is normal but 2 or more of the other 3 values are high, gestational diabetes is diagnosed.

Patients that are Rh negative will need to receive Rhogam around this time. We usually order the blood tests needed to prepare the Rhogam at the same time as the glucose tolerence test to save the patient a blood draw.