Oral Glucose Tolerance Test

An oral glucose tolerance test (OGTT) is a blood test primarily used to diagnose gestational diabetes, a form of diabetes that can affect a woman when she is pregnant. OGTT can also be used to diagnose type 2 diabetes. OGTT measures the level of glucose (a type of sugar) in your blood at different time intervals (2 hours for diagnosis of type 2 diabetes) after you drink a solution containing glucose.1

Who is at risk for gestational diabetes?

Gestational diabetes can be caused by the hormones that are released by the body during pregnancy that reduce insulin production. Like type 2 diabetes, being overweight or obese, having a family history of type 2 diabetes, fat distribution in the waist-to-hip area, a sedentary lifestyle, and smoking can all increase risk for gestational diabetes. In addition to these, having a previous large baby (>9 lbs) also increases the risk for gestational diabetes.2

Why is it important to measure blood glucose?

Our bodies require energy to function properly and we get that energy from the foods we eat. Our diet (everything we eat and drink) includes three main sources of energy (also known as calories): protein, fat, and carbohydrates (sugars, starches, and fibers).When the body digests most sources of carbohydrates, they are transformed through digestion into a very important source of instant energy, a form of sugar called glucose. Our bodies depend on the action of a number of different natural body chemicals called hormones, including insulin, amylin, incretins, and glucagon, working together in conjunction, to control how we use glucose. In type 2 diabetes, these hormones no longer work in the way they should and this results in elevated blood glucose. Elevated blood glucose that persists over time can lead to serious health complications. If you have gestational diabetes and it goes undetected and untreated, it can result in complications such as giving birth to a baby with high birth weight (greater than 9 lbs). It can also increase your risk for preeclampsia and increase the risk of obesity later in life for your child.2-6

How is the OGTT done and what do the results mean?

Diagnosing type 2 diabetes. In OGTT, a sample of your blood is drawn after you drink a special concentrated glucose drink. For diagnosis of type 2 diabetes, the OGTT is done 2 hours after taking a 75-gram glucose drink.1

Oral glucose tolerance test results3

Normal

Impaired

Diabetes**

Oral glucose tolerance*

Less than 140 mg/dL 140 to 199 mg/dL 200 mg/dL or higher

*Result 2 hours after oral glucose test.

**For a diagnosis of diabetes, results of the oral glucose tolerance test must be confirmed by repeating the test a second

time.

If your blood glucose level is 200 mg/dL or higher you may have diabetes and a second test is done to confirm this. If your blood gluose level is in the range of 140 mg/dL to 199 mg/dL, you are considered to have prediabetes.1

Diagnosing gestational diabetes. For diagnosis of gestational diabetes, OGTT can be done using a one- or two-step approach and is usually done at 24 to 28 weeks into your pregnancy.1

In the one-step approach, a blood glucose reading is taken in the morning after fasting overnight (at least 8 hours). After this initial fasting glucose test, a 75-gram glucose drink is taken and blood glucose is tested at 1 and 2 hours after taking the glucose drink. A diagnosis of gestational diabetes is made if your blood glucose is elevated for any of the three tests:

  • Fasting: 95 mg/dL
  • 1 hours after glucose drink: 180 mg/dL
  • 2 hours after glucose drink: 155 mg/dL
  • 3 hours after glucose drink: 140 mg/dL

In the two-step approach, the first step is a non-fasting blood glucose test 1 hour after taking a 50-gram glucose drink. If this initial test is positive (130 mg/dL to 140 mg/dL, depending on individual factors), the second step (done on a separate visit) is to measure blood glucose reading taken after fasting overnight (at least 8 hours), then at 1, 2 and 3 hours after taking a 100-gram glucose drink. A diagnosis of gestational diabetes is made if your blood glucose is at or above the following levels in two of the four tests:

  • Fasting: 95 mg/dL
  • 1 hours after glucose drink: 180 mg/dL
  • 2 hours after glucose drink: 155 mg/dL
  • 3 hours after glucose drink: 140 mg/dL

Another group of recommendations by the National Diabetes Data Group uses thresholds for diagnosing gestational diabetes that are somewhat higher than those shown above.

Written by: Jonathan Simmons | Last reviewed: May 2014.
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