Oral Glucose Tolerance Test

Updated On January 16, 2023 | by Steffi Stark

Glucose Tolerance Test

The Glucose Tolerance Test (GTT) or Oral Glucose Tolerance Test (OGTT) is a method for diagnosing insulin resistance or diabetes mellitus in a person. It is a more significant indicator of diabetes as compared to normal finger prick testing.

What is meant by an OGT test?

This test is done to determine whether a person’s body experiences any trouble while metabolizing the consumption of glucose or carb. In this test, the patient is given a glucose drink and then the measurement of blood sugar level is done before and at intervals following the consumption of a glucose drink.

Why is This Test Done?

The OGTT measures how the body is capable of managing sugar levels after a meal. Glucose is a type of sugar formed when carbs (taken from the foods) get broken down by the body. Few of this sugar is utilized for the purpose of energy; while the rest is stored for future use. Insulin and glucagon are two hormones that control the amount of sugar in a person’s blood. If a person has an excess of these, his or her pancreas secretes insulin in order to assist the cells in storing and absorbing the sugar. If a person has slight of these hormones, the pancreas secretes glucagon to facilitate the release of this stored sugar back into the bloodstream. Under standard conditions, the body would be capable of sustaining an ideal balance of blood sugar. On the other hand, if body parts are weakened, sugar starts accumulating rapidly giving rise to high blood glucose (hyperglycemia) or diabetes.

It is a highly sensitive test that is done to detect abnormalities that other tests miss. OGTT is recommended for the following purposes:

  • Screening and diagnosis of prediabetes
  • Screening and diagnosis of gestational diabetes
  • Screening and diagnosis of type 2 diabetes
  • Diagnosis of beta-cell dysfunction (there is no secretion of insulin)
  • Diagnosis of reactive hypoglycemia (a condition in which the glucose levels fall after consuming food)
  • Diagnosis of some rare problems that have an impact on the metabolism of carbs (for example, hereditary fructose intolerance)
  • Diagnosis of acromegaly (a condition in which the pituitary gland becomes overactive).

How is the OGTT Test Done?

  • A person is asked not to have anything, or drink certain fluids, for a maximum of 8 to 12 hours prior to the test. Also, a person is asked not to take any medicines before the test, only as per the doctor’s guidance.
  • A person’s blood is taken initially to evaluate his or her blood glucose level before the test. 
  • The next step is to consume a very sweet relishing sugar drink.
  • Next, blood samples are taken at regular intervals of half an hour or one hour or a single test after 2 hours.
  • This test can take up to 3 hours. 

Types of OGTT Test

The OGTT method may differ significantly depending upon the objectives of the test. The amount of the oral glucose solution may also vary with the timing and number of blood draws needed. Even, there may be differences in which a low-carb meal can be suggested.

2 standard variations can be utilized for screening and diagnostic purposes:

  • A 2-hour OGTT, in which there are two blood draws. This test diagnoses diabetes or prediabetes in non-pregnant adults as well as children.
  • A 3-hour OGTT, in which there are four blood draws. This test screens gestational diabetes.

What Should the OGTT Results Be?


  • Fasting value (prior to test): under 6 mmol/L
  • At 2 hours: under 7.8 mmol/L

People having an impaired glucose tolerance (IGT)

  • Fasting value (prior to test): 6.0 to 7.0 mmol/L
  • At 2 hours: 7.9 to 11.0 mmol/L

For people with diabetes

  • Fasting value (prior to test): over 7.0 mmol/L
  • At 2 hours: over 11.0 mmol/L

What Do These Test Outcomes Indicate?

If a person lies within the impaired glucose tolerance range, he or she is expected to be suggested to adopt certain lifestyle alterations like sleep deprivations. In certain cases, blood sugar lowering drugs can be recommended. If a person falls within the diabetic range, it is almost certain that he or she would be recommended anti-diabetic drugs to help his or her body keep their blood sugar levels down.

Pros and Cons of OGTT Test

  • OGTT is far more sensitive as compared to the fasting plasma glucose test (FPG) and is frequently done when there is a doubt regarding diabetes, however, the FPG returns a normal result.
  • OGTT is the only test that may conclusively diagnose IGT.

Also, the OGTT has its limitations:

  • It is a time-consuming test, needs an extensive pre-test fasting as well as prolonged testing and waiting period.
  • The test outcomes might be affected by factors such as other diseases, medicines, or stress.
  • In terms of precision, OGTT gives a sensitivity of 81%-93%, which is far better than the FGP, which gives a sensitivity of 45%-54%.
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