Glucose Tolerance Test – What You Should Know

When a doctor wants to test a patient for diabetes, the first diagnostic examination recommended is a glucose tolerance test. Though heard often, not many know what this test is all about, what it reveals and how to interpret the results. This article aims to provide you with certain basic information about this important procedure.

What is the test?

It is a laboratory method of testing how the body metabolizes or breaks down glucose. It shows how soon the sugar, administered orally is cleared from the blood.  The test is recommended for testing a patient for diabetes, insulin resistance and more.

The test is also called OGTT or Oral Glucose Tolerance Test, because a standard dose of glucose is given orally and the blood sample is taken after 2 hours after such ingestion.

Glucose tolerance test: before, during and after

Since the ‘70s, standard OGTT follows certain guidelines regarding the dose of glucose to be administered and duration.  Before this test is done, certain pre-test procedures are followed. These are:

  • No restriction on carbohydrate intake in the days and weeks prior to the test.
  • Fasting is necessary (can have water) 8 to 12 hours before blood sample is taken.
  • The test is not recommended if the patient is suffering from any illness as the results can misrepresent actual glucose metabolism
  • This test is usually done in the morning.
  • The procedure involves drinking a liquid that contains 75 grams (WHO recommendation for adults) of glucose. Blood sample is taken before drinking as well as every 30 or 60 minutes after ingestion of the solution. The test usually takes up to 3 hours.

The number of blood samples and intervals actually depend on the purpose of the glucose tolerance test. When the main purpose is to simply check for diabetes, the most important blood sample is drawn after 2 hours and this could be the only sample collected. However, some physicians may request multiple samples, which are then taken in a span of 3 hours.

What are the purposes of this test?

This is one of the most trustworthy tools for diagnosing diabetes. It is also used to screen pregnant women for gestational diabetes. Even when the fasting blood sugar levels are normal, doctors use the glucose tolerance test if diabetes is suspected.

How to interpret the results?

Glucose levelsNORMALImpaired glucose tolerance(IGT)Diabetes Mellitus (DM)
Venous PlasmaFasting2hrsFasting2 hrsFasting2 hrsFasting2 hrs
(mg/dl)<110<140>110 & <126<140<126 >140>126>200


Impaired glucose tolerance is also called ‘pre-diabetes’ condition. This means that the patient has an increased risk of developing diabetes, whereas, glucose level of 200 mg/dL indicates onset of type 2 diabetes.

What are the factors that may impact the results?

The results of the glucose tolerance test may be affected by factors like post-surgical stress, strenuous exercise, consumption of certain drugs like atypical antipsychotic medications, Beta-blockers, corticosteroids, estrogens, oral contraceptives, etc.