Glucose Tolerance Test – Why & How

The glucose tolerance test identifies abnormalities in the way your body handles glucose after a meal — often before your fasting blood glucose level becomes abnormal. The test can be used to screen for type 2 diabetes.

Test Preparation

It’s important to eat and drink normally in the days leading up to the glucose tolerance test. Let your doctor know if you’re ill or taking any medications, as these factors can affect the results of your test.

For eight hours before the test, you won’t be able to eat or drink anything. You might want to fast overnight and schedule the test for early the following morning.


Results of the oral glucose tolerance test are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

If you’re being tested for type 2 diabetes, two hours after drinking the glucose solution:

  • A normal blood glucose level is lower than 140 mg/dL (7.8 mmol/L).
  • A blood glucose level between 140 mg/dL and 199 mg/dL (7.8 and 11 mmol/L) is considered impaired glucose tolerance, or prediabetes. If you have prediabetes, you’re at risk of eventually developing type 2 diabetes. You’re also at risk of developing heart disease, even if you don’t develop diabetes.
  • A blood glucose level of 200 mg/dL (11.1 mmol/L) or higher may indicate diabetes.

If the results of your glucose tolerance test indicate type 2 diabetes, your doctor may repeat the test on another day or use another blood test to confirm the diagnosis. Various factors can affect the accuracy of the glucose tolerance test, including illness, activity level and certain medications.

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