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What is GTT?

Glucose tolerance test (GTT), commonly known as oral glucose tolerance test, is a common diagnostic test for diabetes, especially in pregnant women and individuals with borderline diabetes. GTT checks the level of glucose in blood in a given time period and concludes whether the body is capable of absorbing it.

Glucose level in the blood is kept constant at all times by the body. Post-meal glucose levels may increase by 50%; however, they go back to normal in about 2 hours. When a person is affected by diabetes mellitus, their body is unable to absorb the glucose generated from food. Consequently, glucose levels remain high in their blood frequently. Increased glucose levels in blood can damage blood vessels and impair body functions, thus increasing the risk of other conditions such as loss of vision and damage to the nervous system.

  1. Why is GTT performed?
  2. How do you prepare for GTT?
  3. How is GTT performed?
  4. What do GTT results mean?

A healthcare provider will recommend a GTT under the following circumstances:

If results from other diabetes tests are indeterminate, an individual may be asked for GTT as it is a gold standard for such cases. Doctors recommend a yearly check for diabetes in high-risk patients to eliminate any further risks.

During pregnancy, a woman’s body undergoes several metabolic adjustments for the growing foetus. Therefore, GTT is a standard test in the 3rd trimester of pregnancy. It is performed to eliminate the risk of gestational diabetes that might affect mother’s health.

Maintaining regular food intake for 3 days preceding the test is necessary for GTT. The doctor should be informed about any existing medical conditions or intake of any medications or herbal supplements. Do not change the course of any medication without consulting your doctor. Pregnant women should consult the doctor to get further instructions for this test.

Non-pregnant individuals:

Individuals should fast for about 8-10 hours before the test. During this time, he/she can drink water, but food, juices and caffeine should not be taken before the test. Smoking should also be avoided during fasting hours. Generally, GTT is performed in the morning.

Pregnant women:

GTT can be performed at any time during the day for pregnant women. In exceptional conditions, the doctor may ask to follow the standard preparation instructions that are recommended for non-pregnant individuals.

GTT protocols for pregnant women and non-pregnant individuals differ slightly.

Non-pregnant individuals:

  • A blood sample is collected from the arm using a sterile needle after swabbing the concerned area with an appropriate antiseptic solution. Some people experience a little discomfort at needle insertion site, which disappears soon after the test.
  • A sugary drink is given to the individual, which they should consume immediately.
  • There is a waiting period of 2 hours after that, during which, eating, drinking any other juices or caffeinated beverages, and smoking is not permitted.
  • Another blood sample is collected at the end of 2 hours.

Pregnant women:

  • The pregnant woman is given a glass of sugary drink, which she should consume immediately.
  • After a waiting period of 1 hour, a blood sample is collected from her arm using a sterile needle. Eating or drinking any juices or fluids is not permitted during the waiting period.
  • In case of any blood sugar abnormalities in test results, the doctor may require another test where they would follow the same procedure as that for non-pregnant individuals.

Normal results:

  • In non-pregnant individuals, typically, fasting blood sugar is below 6 mmol/L and the values 2 hours after drinking a sugary drink can go up to 7.8 mmol/L. These results mean that the body can absorb glucose normally, and the person does not have diabetes.
  • In pregnant women, the typical value would be either equivalent to or below 7.8 mmol/L after 1 hour of drinking the sugar water.

Abnormal results:

Higher than normal results could indicate either an early sign of diabetes or presence of clinical diabetes.

In case of non-pregnant individuals, glucose levels between 7.8-11.1 mmol/L 2 hours after sugar intake are an indication of a pre-diabetic condition. This would mean that the individual is at risk for diabetes and should get themselves checked yearly.

A value higher than 11.1 mmol/L is an indication of diabetes and requires immediate attention of a medical practitioner.

In pregnant women, a value of more than 7.8 mmol/L could indicate gestational diabetes. The doctor may order a repeat test before beginning an appropriate treatment.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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References

  1. Metzger BE. Diabetes mellitus and pregnancy. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016. Chap 45.
  2. American Pregnancy Association: Glucose Tolerance Test
  3. American Diabetes Association. Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes—2018. Diabetes Care. 2018 Jan; 41(Suppl 1):S13-S27.
  4. Australian Family Physician [internet]: The Royal Australian College of General Practitioners, East Melbourne, Victoria; Oral glucose tolerance testing
  5. Guyton A.C, Hall J.E. Textbook of medical physiology. 11th edition. Pennsylvania: Elsevier Saunders; 2016. Chapter 78, Insulin, Glucagon, and Diabetes Mellitus; p. 961-977.
  6. Landon MB, Catalano PM, Gabbe SG. Diabetes mellitus complicating pregnancy. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 7th ed. Philadelphia, PA: Elsevier Saunders; 2017. Chap 40.
  7. Committee on Practice Bulletins--Obstetrics. Practice Bulletin No. 190: Gestational diabetes mellitus. Obstet Gynecol. 2018;131(2):e49-e64.
  8. Provan D, Oxford Handbook of Clinical and Laboratory Investigation. 4th ed. United Kingdom: Oxford University Press; 2018. Chapter 2, Endocrinology and Metabolism; p.194-198.
  9. Eyth E, Basit H, Smith CJ. Glucose Tolerance Test. [Updated 2019 Apr 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019
  10. National Institute of Diabetes and Digestive and Kidney Diseases [internet]: US Department of Health and Human Services; Diabetes Tests & Diagnosis