Glucose Tolerance Test Preparation, Normal Range, Diabetes, and Pregnancy

Abstract

The glucose tolerance test (GTT) is a diagnostic tool used to assess how efficiently the body processes glucose, the primary sugar in the bloodstream. This comprehensive guide aims to provide patients, their families, and the general public with detailed information about the GTT, including its purpose, procedure, and interpretation of results. By understanding the significance of glucose regulation and the role of the GTT, individuals can take proactive steps in managing their health, particularly in relation to conditions like diabetes and gestational diabetes.​ This is part of the 'Public Education Series' initiative by Exon Publications.

Cite as: Glucose Tolerance Test: Preparation, Normal Range, Diabetes, and Pregnancy. Brisbane (AU): Exon Publications; 2025. Published on  28 Mar. DOI: https://doi.org/10.36255/glucose-tolerance-test-preparation-normal-range-diabetes-pregnancy

Introduction

Maintaining appropriate blood sugar levels is essential for overall health. The glucose tolerance test is a valuable method for evaluating the body's ability to metabolize glucose and is commonly employed to detect diabetes, insulin resistance, and other metabolic disorders. This article offers an in-depth exploration of the GTT, its importance, and what individuals can anticipate before, during, and after the test (1-7).

The glucose tolerance test measures the body's response to glucose over a set period. It involves fasting overnight, consuming a glucose-rich beverage, and undergoing multiple blood draws to monitor how blood sugar levels change over time. Image: Depiction of blood in a tube for Glucose Tolerance Test. Image Credit: someone25 from Getty Images via Canva.com

What is Glucose and Why Does it Matter?

Glucose is a simple sugar that serves as the primary energy source for the body's cells. It is derived from the foods consumed and is transported via the bloodstream to various tissues. Proper regulation of blood glucose levels is crucial, as imbalances can lead to health issues. Elevated blood glucose levels may suggest diabetes or insulin resistance, while low levels can result in hypoglycemia, causing symptoms such as dizziness, confusion, and fainting.​

Normal and Fasting Glucose Ranges

For individuals without diabetes, normal fasting blood glucose levels typically range from 70 to 99 milligrams per deciliter (mg/dL) [3.9 to 5.5 mmol/L]. After consuming food, blood sugar levels may rise but generally remain below 140 mg/dL [7.8 mmol/L] two hours post-meal. Fasting blood glucose levels between 100 and 125 mg/dL [5.6 to 6.9 mmol/L] may indicate prediabetes, while levels of 126 mg/dL [7.0 mmol/L] or higher on multiple occasions may suggest diabetes. ​

What is Glucose Tolerance Test?

The glucose tolerance test measures the body's response to glucose over a set period. It involves fasting overnight, consuming a glucose-rich beverage, and undergoing multiple blood draws to monitor how blood sugar levels change over time. Healthcare providers use this test to identify abnormalities in glucose metabolism, which may indicate conditions such as diabetes or prediabetes. ​

Who Needs the Glucose Tolerance Test?

Healthcare providers may recommend a glucose tolerance test for individuals at risk of developing diabetes or those exhibiting symptoms such as increased thirst, frequent urination, unexplained weight loss, or fatigue. Pregnant women are often screened for gestational diabetes between 24 and 28 weeks of pregnancy using a version of the GTT. Individuals with risk factors including obesity, a sedentary lifestyle, a family history of diabetes, or a history of gestational diabetes may also be advised to undergo the test. ​

Types of Glucose Tolerance Tests

There are several types of glucose tolerance tests, each tailored to specific medical needs.

Oral glucose tolerance test

The most common is the oral glucose tolerance test, which involves drinking a glucose-rich beverage and measuring blood sugar levels over time through blood draws. This test is widely used to detect type 2 diabetes and prediabetes.

Intravenous glucose tolerance test

Another version is the intravenous glucose tolerance test, where glucose is delivered directly into a vein and blood samples are taken at intervals. This form is used less often and may be reserved for research or complex medical cases.

Glucose Tolerance Test in Pregnancy

During pregnancy, the glucose tolerance test is used to screen for gestational diabetes, a condition characterized by elevated blood sugar levels that develop during pregnancy and typically resolve after childbirth. The test is usually performed between 24 and 28 weeks of gestation. In this case, either a one-step or two-step method is used. The one-step test involves fasting, drinking a glucose drink, and checking blood sugar at set times. The two-step test begins with a shorter screening, followed by a longer diagnostic test if the initial results are high. Each test serves the same core purpose of measuring how the body handles glucose, but the approach may vary depending on the individual's condition and risk factors.

Glucose Tolerance Test Preparation: Food, Fasting, and Medications

Proper preparation is necessary to ensure the glucose tolerance test gives accurate results.

Food

In the days leading up to the test, individuals are generally advised to follow their normal diet and activity levels. It is not recommended to go on a low-carbohydrate or restrictive diet before the test, as this can affect the body's glucose response.

Fasting

Fasting is required for at least eight hours before the test, meaning no food or drink other than water during that time.

Medications

On the day of the test, the individual should inform the healthcare provider about all medications, including over-the-counter and herbal supplements, as some may influence blood sugar levels. These might include corticosteroids, diuretics, or beta-blockers. Some medications may need to be paused or adjusted before the test, but only under medical supervision. Following all preparation instructions closely helps ensure that the results of the test reflect the body’s true ability to handle glucose.

Glucose Tolerance Test Procedure

On the day of the glucose tolerance test, the individual arrives at the clinic or laboratory after fasting for at least eight hours. The healthcare provider first confirms the fasting period, checks any medications taken, and may ask about symptoms or relevant history.

A baseline blood sample is collected to measure fasting glucose levels. The person is then given a sweet glucose drink, which may taste syrupy or artificially flavored. This drink contains a set amount of glucose, usually 75 grams [2.6 ounces] for adults or a different amount as prescribed in pregnancy or for children. After drinking the solution, the individual must stay at the testing site without eating, drinking anything other than water, or engaging in physical activity. Blood samples are taken at regular intervals—usually after one hour and two hours—to monitor the body's response.

The procedure may take two to three hours in total. Some people may feel lightheaded, nauseated, or tired during the process due to the high sugar intake, especially if they have underlying insulin resistance or diabetes. Once the final sample is taken, individuals can resume normal eating and activities unless instructed otherwise. The results are typically available within a few days and are reviewed with a healthcare provider.

Glucose Tolerance Test Results

The glucose tolerance test results show how the body manages blood sugar over time. These results are evaluated against established thresholds to determine if glucose levels fall within a healthy range. A normal response typically shows a blood glucose level below 140 mg/dL [7.8 mmol/L] two hours after drinking the glucose solution. A reading between 140 and 199 mg/dL [7.8 to 11.0 mmol/L] is considered impaired glucose tolerance, which may indicate prediabetes. A level of 200 mg/dL [11.1 mmol/L] or higher at the two-hour mark is consistent with diabetes and often leads to a diagnosis, especially when confirmed with repeat testing.

In the case of gestational diabetes, different thresholds apply. For example, in a two-step test, glucose levels are measured at fasting, one hour, and two hours, with specific cutoffs for each. If one or more values are above the recommended range, a diagnosis of gestational diabetes may be made. It is essential for patients to review results with their healthcare provider, as interpretation depends on the full clinical picture, not just the numbers alone.

Limitations of Glucose Tolerance Test

While the glucose tolerance test is a useful tool, it does have some limitations. One of the main concerns is variability. Blood glucose responses can fluctuate due to stress, illness, sleep deprivation, or certain medications, which may lead to false positives or false negatives.

The test requires a long time commitment and multiple blood draws, which may not be feasible for all individuals. Some people may find the glucose drink unpleasant or experience side effects like nausea or dizziness. Additionally, the test may not be suitable for people with severe gastrointestinal issues or those unable to tolerate high sugar loads.

It is also important to note that the glucose tolerance test measures how the body handles sugar at one point in time, which may not reflect daily blood sugar patterns. Other tests, such as fasting glucose, hemoglobin A1C, or continuous glucose monitoring, may provide complementary information. Furthermore, the test cannot distinguish between different causes of elevated blood glucose, such as insulin resistance, beta-cell dysfunction, or genetic factors like mutations in the GCK or HNF1A genes seen in MODY (maturity-onset diabetes of the young).

For these reasons, the glucose tolerance test should be interpreted in context and may be combined with other tests for a complete evaluation.

Conclusion

The glucose tolerance test is a well-established method for measuring how efficiently the body processes sugar. It is particularly helpful for diagnosing diabetes, prediabetes, and gestational diabetes. While not perfect, the glucose tolerance test remains a valuable tool for detecting early signs of glucose metabolism disorders.

References

1. American Diabetes Association. 2. Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes-2020. Diabetes Care. 2020 Jan;43(Suppl 1):S14-S31. https://doi.org/10.2337/dc20-S002


2. World Health Organization. Definition and Diagnosis of Diabetes Mellitus and Intermediate Hyperglycemia: Report of a WHO/IDF Consultation. Geneva: WHO; 2006. Available from: https://www.who.int/diabetes/publications/diagnosis_diabetes2006/en/.


3. Sacks DB, Arnold M, Bakris GL, Bruns DE, Horvath AR, Kirkman MS, et al. Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus. Diabetes Care. 2011 Jun;34(6):e61-e99. https://doi.org/10.2337/dc11-9998


4. International Association of Diabetes and Pregnancy Study Groups Consensus Panel. International Association of Diabetes and Pregnancy Study Groups Recommendations on the Diagnosis and Classification of Hyperglycemia in Pregnancy. Diabetes Care. 2010 Mar;33(3):676-82. https://doi.org/10.2337/dc09-1848


5. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. Obstet Gynecol. 2018 Feb;131(2):e49-e64. https://doi.org/10.1097/AOG.0000000000002501


6. Nathan DM, Davidson MB, DeFronzo RA, Heine RJ, Henry RR, Pratley R, et al. Impaired Fasting Glucose and Impaired Glucose Tolerance: Implications for Care. Diabetes Care. 2007 Mar;30(3):753-9. https://doi.org/10.2337/dc07-9920


7. Matsuda M, DeFronzo RA. Insulin Sensitivity Indices Obtained from Oral Glucose Tolerance Testing: Comparison with the Euglycemic Insulin Clamp. Diabetes Care. 1999 Sep;22(9):1462-70. https://doi.org/10.2337/diacare.22.9.1462

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This article is part of the 'Public Education Series' initiative by Exon Publications. It was written by professional medical writers for the general public in plain language, based on peer-reviewed articles indexed in PubMed, and further reviewed for scientific accuracy by experts. The views and opinions expressed in this article are believed to be accurate at the time of publication. However, the publisher, editors, and authors cannot be held responsible or liable for any errors, omissions, or consequences arising from the use of the information provided. The publisher makes no warranties, explicit or implicit, regarding the contents of this article or its use. The information in this article is intended solely for informational purposes and should not be considered medical advice.