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Glucose Tolerance Test

What is the Glucose Tolerance Test (GTT)?

The GTT is a blood-draw test used for Gestational Diabetes screening.  This can be done to measure blood sugar levels in your blood, in several forms:

  • Fasting GTT – This is the most accurate.  Blood is drawn after a 12 hour fast.  (Usually first thing in the morning.)
  • 1 Hour GTT – Blood is drawn 1 hour after drinking a 50mg glucose drink.
  • 3 Hour GTT – If the 1 hour GTT comes back elevated, then a 3 hour GTT will usually be ordered.  They will test your fasting blood sugar levels, have you drink 75-100mg of glucose, test again at 1 hour, 2 hours, and 3 hours.

What is the normal range?

Something that you should know about the GTT “normal ranges”, is that even though normal non-diabetic women’s blood sugar NORMALLY runs higher, the levels used to “discover” diabetes in pregnancy are actually set LOWER than when not pregnant!  While pregnant, a woman’s body naturally takes a bit longer to break down sugar in the blood and process it.

The American Diabetes Association (ADA) has these values set for normal range in adults for blood sugar levels at each interval.

  • Fasting < 115
  • 1 hour < 200
  • 2 hours < 140

The National Diabetes Data Group (NDDG):

  • Fasting < 105
  • 1 hour < 190
  • 2 hours < 165
  • 3 hours < 145
1999 WHO Diabetes criteria – Interpretation of Oral Glucose Tolerance Test
Glucose levels NORMAL IFG IGT DM
Venous Plasma Fasting 2hrs Fasting 2hrs Fasting 2hrs Fasting 2hrs
(mmol/l) <6.1 <7.8 > 6.1 & <7.0 <7.8 <7.0 >7.8 >7.0 >11.1
(mg/dl) <100 <140 >100 & <126 <140 <126 >140 >126 >200

IFG – Impaired Fasting Glycaemia, IGT – Impaired Glucose Tolerance, DM – Diabetes Mellitus

Both of these are acceptable charts for determining whether or not a woman has Gestational Diabetes.  Always ask what your numbers were for each of these intervals!

Should every pregnant woman have the GTT done?

According to The American College of Obstetrics and Gynecology (ACOG), NO.  The ACOG recommends the GTT screening for women who has two or more of these factors:

  • Over 25 years old
  • Morbidly Obese
  • Strong family history of Diabetes Mellitus
  • In a high risk ethnic group (Hispanic, African American, Native American, or Pacific Islander)
  • History of abnormal glucose tolerance
  • A stillbirth or very large baby in a previous pregnancy

However, this criteria is controversial, and you will likely come across many care providers who will recommend that EVERY pregnant woman be screened.  It is up to each individual woman to weigh the benefits and risks of prenatal testing.  Ask yourself these important questions:

Do I have factors that would make me high risk for a positive result on this test?
What tests, if any, will be done if the first comes back positive?
What will this mean for the rest of my prenatal care?
Will it leave the door open to unnecessary intervention either in pregnancy or labor?
What are the benefits, if any, to having this test done?
What will happen, if anything, if I refuse this test?

Please visit Kmom’s Site about Gestational Diabetes screening!

Websites for Glucose Tolerance Level Values:

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