Glucose Tolerance Test: Purpose, Preparation, Procedure & Normal Range of Results

Glucose tolerance test is done to measures how efficiently your body produces insulin. This test should be done if you are at high or average risk of gestational diabetes.

Updated: June 16, 2022


Usually this test is useful in diagnosing gestational diabetes. Gestational diabetes develops during pregnancy (gestation). It causes blood sugar levels to become too high like Type 1 and Type 2 diabetes. Gestational diabetes needs to be recognized and treated quickly because it can cause health problems for both mother and baby. 
Your digestive system breaks down most of the food you consume into a sugar called glucose. A hormone called insulin is produced from your pancreas which help absorbing glucose from your blood to work as fuel for muscle, fat, and other cells. But if the pancreas is unable to produce enough insulin or if the cells have a problem responding to it, too much glucose remains in your blood instead of moving into cells and getting converted to energy.
In pregnancy your body needs additional insulin to process excess glucose in blood. The pancreas secretes more insulin as the body demands more.But if the pancreas can't keep up with the increased demand for insulin during pregnancy, blood sugar levels rise too high because the cells aren't using the glucose resulting in gestational diabetes.

When GTT test should be done?

Glucose tolerance test is done to measures how efficiently your body produces insulin. 
This test should be done if you are at high or average risk of gestational diabetes. If you were obese at the start of your pregnancy, you had gestational diabetes during a previous pregnancy, or you have a mother, father, sibling or child with diabetes then you are at a high risk of gestational diabetes and this test is advisable to do at your first prenatal visit to the doctor. Sometime during your second trimester, typically between 24 and 28 weeks of pregnancy you are likely to have this test done if you are at average risk of gestational diabetes. 

How glucose tolerance test is done?

Initially a glucose challenge test will be done by drinking a syrupy glucose solution. You will have a blood test to measure your blood sugar level after one hour. A blood sugar level less than 140 mg/dL or 7.8 mmol/L is considered as normal. You have a higher risk of gestational diabetes if your blood sugar level is higher than normal. In this case a follow-up test will be advised to determine if you have gestational diabetes.
This test is done after an overnight fasting. At the beginning of this test the fasting blood sugar level is measured by drawing a blood sample. Then you will be given a sugary liquid to drink which contain a higher concentration of glucose and blood sugar levels are tested periodically for the next two to three hours. 

Result of glucose tolerance test:

A reading of more than 200 mg/dL or 11.1 mmol/L after two hours indicates diabetes. A reading between 140 and 199 mg/dL (7.8 mmol/L and 11.0 mmol/L) indicates prediabetes.
If any of the blood sugar readings are higher than the normal values, then you will be diagnosed with gestational diabetes.

When Blood Is DrawnPrediabetesDiabetesGestational Diabetes
Fasting100-125 mg/dLgreater then 126 mg/dLgreater then 95 mg/dL
After 1 Hourgreater then 180 mg/dL
After 2 Hours140-199 mg/dLgreater then 200 mg/dLgreater then 155 mg/dL

Who should go for a diabetes screening?

The following people are recommended to go for a diabetic screening:
  • Anyone with a body mass index higher than 25 regardless of age or who is obese
  • who has additional risk factors, such as high blood pressure
  • who has a sedentary lifestyle
  • anyone with a history of polycystic ovary syndrome
  • any women who has delivered a baby weighed more than 9 pounds
  • women with a history of diabetes in pregnancy
  • anyone who has high cholesterol levels
  • people with a history of heart disease
  • people having a close relative with diabetes
In addition anyone older than age of 45 is advised to do an initial blood sugar screening.If the results are normal, the screening can be repeated every three years thereafter.


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