What is the 3-Hour Glucose Tolerance Test GTT?
The 3-hour glucose tolerance test is used during pregnancy to rule out or diagnose gestational diabetes in what's called the "2-step" approach.

ACOG supports the "2-step" approach to diagnosing diabetes in pregnancy

1. Step 1: 24 – 28 week 1 hour venous glucose measurement following 50g oral glucose solution (1 hour GCT)
2. Step 2: a 3 hour oral glucose tolerance test (3-hour OGTT) if step 1 is positive and the GCT was elevated

Note: Diagnosis of GDM is based on 2 abnormal values on the 3 hour OGTT
ACOG recommends that currently there is insufficient evidence to diagnose GDM based on only one abnormal value
Patients with only one elevated value may require additional surveillance

Typically, the 1-hour glucose tolerance test is given first with the 3-hour glucose tolerance test reserved for women who fail the initial test. 

Activity and diet need to be normal for at least three days prior to the test. Women fast the night before reporting to the lab. Upon arrival, a fasting blood draw is performed. After the initial blood draw, 100 grams of glucose solution are consumed. One, two and three hours after the glucose solution has been consumed, lab techs draw another sample of blood. If at least two of the normal values are exceeded during the test, gestational diabetes is diagnosed.

Normal Value Range

• Fasting Values: 95 mg/dL or 5.3 mmol/L
• 1 Hour Values: 180 mg/dL or 10 mmol/L
• 2 Hour Values: 155 mg/dL or 8.6 mmol/L
• 3 Hour Values: 140 mg/dL or 7.8 mmol/L

< Laboratory Values During Pregnancy