The glucose tolerance test is a method to judge how well your body is able to metabolize glucose.

Glucose is the type of sugar your body uses to supply energy to its cells. Your body breaks down the food you eat into glucose that then enters your bloodstream. A hormone called insulin, made by your pancreas helps bring the glucose from your bloodstream into the cells of your body.

If your body’s cells have difficulty accepting insulin, a condition called insulin resistance, or if your body cannot produce enough insulin, the glucose levels in your blood will be too high. High blood glucose can be a sign of prediabetes or diabetes. Diabetes is a serious medical issue that can damage the organs and tissues of the body if it is not controlled.

By measuring how your body processes glucose, the glucose tolerance test can be used with other tests to detect or confirm prediabetes and diabetes.

Purpose of the test

The glucose tolerance test is one way to assess how well your body metabolizes glucose. Your doctor may have several different reasons for recommending this test:

• Diagnosing prediabetes and diabetes: Diagnosis is the process of determining what is causing changes in your health that you or your doctor may have observed. The glucose tolerance test may be ordered if you are showing signs or symptoms of diabetes, especially if other tests for diabetes are inconclusive.
• Screening for prediabetes and diabetes: Screening is checking for a disease before it causes signs and symptoms. Your doctor may want you to be screened for diabetes if you are over 45 years old or are at a greater-than-average risk of the disease. In general, other types of diabetes tests are more commonly used for screening than the glucose tolerance test. However, the glucose tolerance test is frequently used to screen for diabetes in pregnant women.
• Screening for and diagnosing gestational diabetes: Gestational diabetes is a form of diabetes that can develop in women while they are pregnant. If you are pregnant, your doctor may suggest this testing, which is described in our guide to Glucose Tests for Gestational Diabetes.

What does the test measure?

The glucose tolerance test measures the amount of glucose that remains in your bloodstream after fasting and then after drinking a sugary drink at fixed intervals. Blood glucose levels are typically measured in milligrams per deciliter, or mg/dL.

How to get tested

Your doctor is typically the one to order a glucose tolerance test. The test may be conducted in your doctor’s office or in a hospital, clinic, or laboratory.