What is the Glucose Tolerance Test?

Glucose Tolerance Test analyses the ability of a human body to break down sugar or glucose. This test diagnoses the presence of cardiovascular diseases and diabetes, especially type 2 diabetes mellitus and gestational diabetes. The test is also effective in predicting the risk of developing diabetes and cardiovascular disorders in the future. It is also known by another name – GTT. The oral glucose tolerance test is generally the most popular method for the test, although there are other methods as well.

There are two types of the test:

Glucose tolerance test or the full or detailed version of the test
Glucose challenge test or a shorter version of the test

Why is the Glucose Tolerance Test Done?

The glucose tolerance test is advised with the purpose of monitoring how the body reacts and processes glucose after a meal. It evaluates the body’s ability to handle a glucose load. The test showcases whether the patient is able to metabolise a standardised quantity of glucose. The result can be classified into three categories – normal, impaired, and abnormal.

Prerequisites for the Glucose Tolerance Test

This is usually a normal fasting blood test, unless your doctor recommends otherwise. This implies that you have to avoid food and water for a minimum of 8 hours before undergoing the test. However, at times, a non-fasting GTT may also be advised as per the discretion of the doctor.

For the normal fasting GTT, it is generally recommended that you follow your usual diet till 3 days before the test. Or, consume 150 gms of carbohydrate every day. This should continue till approximately 8 hours before the scheduled time for the test. If fasting is not practised for at least 8 hours, the time for the GTT will have to be rescheduled.

Don’t forget to mention to your doctor about the medications that you might be taking currently or may have taken in the recent past. This is because there are some medications that are recognised for interacting with this test. These medications include prescription drugs, non-prescription or OTC (over-the-counter) drugs, vitamins, supplements, and herbals. This implies that such medications adversely affect the accuracy of the results of the oral glucose tolerance test, defeating the purpose of taking the test.

Also, discuss your allergic reactions with the doctor. Knowing this will enable him/her to analyse whether the test is suitable for you. As for the medications, he/she may advise you to temporarily stop taking them for a few days before the test. If you are taking certain prescription medications that may interact with the test, the doctor is likely to advise you to consult the doctor who has prescribed them, for his/her advice.

Understanding the Glucose Tolerance Test Results

Type 1 and type 2 Diabetes

Normal results:

Fasting glucose level: 60 – 100 mg/dL
One-hour glucose level: <200 mg/dL
Two-hour glucose level: <140 mg/dL

Impaired results:

Fasting glucose level: 100 – 125 mg/dL
Two-hour glucose level: 140 – 200 mg/dL

Abnormal results:

Fasting glucose level: >126 mg/dL
Two-hour glucose level: >200 mg/dL

Prediabetes

Normal results: 140 – 199 mg/dL

Gestational Diabetes

Normal results:

Fasting glucose level: <90 mg/dL
One-hour glucose level: <130 – 140 mg/dL
Two-hour glucose level: <120 mg/dL

Abnormal results:

Fasting glucose level: >95 mg/dL
One-hour glucose level: >140 mg/dL
Two-hour glucose level: >120 mg/dL

The normal range of the test results often varies slightly between laboratories.

When should the Glucose Tolerance Test be Repeated?

Repeat GTT is likely to be recommended in the following instances:

Patients with hereditary cardiovascular and diabetic conditions are generally advised to take the test once every 6 months or annually.
Patients with a personal medical history of such conditions are advised to take this test frequently, as per the discretion of the doctor.

Procedure for the Glucose Tolerance Test

The procedures for the shorter and the detailed version of the glucose test vary to some extent. The procedures have been explained below:

Glucose Tolerance Test:

The GTT can be performed orally by inserting a BC-shielded IV catheter or with multiple phlebotomy draws.

For the oral glucose tolerance test, the fasting blood sample will be collected for testing at a laboratory. The exact timing at which the blood sample is taken will be recorded. Next, a glucose solution will be given, determined by your weight but not more than 75 gms. This has to be consumed within a maximum of 5 minutes. This will be repeated two or three more times, with an interval of 30 to 60 minutes each. The time intervals and the number of times the glucose solution is taken vary between laboratories.

In case a BC-shielded IV catheter is being used, the line is usually lushed with a heparin solution or saline after each sample is collected. This is done to ensure the patency of the line. After all the blood samples have been collected, they are sent to the laboratory for microscopic analysis.

Glucose Challenge Test:

For the glucose challenge test or the shorter version of the test, the concentrated glucose solution of about 50 grams mixed in 250 to 300 ml of water is consumed. The blood sample is then collected after an hour and sent to the laboratory for close analysis under a microscope. This test can be conducted at any time of the day.

Glucose Tolerance Test: Why and How is it Done?