Glucose Tolerance Test Procedure: Why It’s Done and What to Expect

Fasting and postprandial blood sugar tests are helpful but may not show the full picture of glucose metabolism. The GTT gives more detailed insight, helping diagnose diabetes, prediabetes and gestational diabetes while assessing long-term glucose control.

This blog covers the GTT procedure, when it’s recommended, preparation, what to expect, its role in pregnancy, postprandial testing, and normal glucose ranges in pregnancy.

The glucose tolerance test (GTT) or oral glucose tolerance test (OGTT), measures glucose-use efficiency. Blood samples are taken at regular intervals after the body is subjected to a measured dose of 75 g of glucose.

This test is to note the effectiveness of glucose in being cleared from the bloodstream. An abnormal or delayed response is symptomatic of failure in glucose regulation, which indicates diabetes, gestational diabetes, or impaired glucose tolerance.

Doctors recommend the procedure of the GTT test in different clinical situations, such as:

• Diagnosing type 2 diabetes when fasting or random blood sugar levels are inconclusive.
• Identifying prediabetes in individuals with high-risk factors such as obesity, family history, or a sedentary lifestyle.
• Detecting gestational diabetes during pregnancy. The 75 g glucose test in pregnancy is a standard recommendation by health authorities.
• Monitoring patients with a history of impaired glucose tolerance or unexplained high blood sugar spikes.

Doctors are likely to give patients certain rules they ought to adhere to.

Fasting Before GTT Test

Patients are normally required to fast before the test (usually 8- 12 hours). Water is acceptable, although no feed that contains any calories should be consumed.

Medicines, Illness, and Their Impact on Results

Some medicines impede glucose metabolism, including corticosteroids, diuretics, or beta-blockers. Glucose responses may also change due to illness and infection, or stress

The procedure of the GTT test is simple but requires time and patience.

Glucose Solution (75 gm Glucose Test Explained)

Patients have to drink a sweetened liquid containing 75 g of glucose, dissolved in water. This solution functions as a highlight test of how the body handles sugar.

Blood Draws and Time Gaps (1 hr, 2hr, 3hr)

Blood is first drawn in a fasting state, then again at intervals, typically 1, 2, and sometimes 3 hours after glucose intake. These readings, along with postprandial values, help assess how quickly blood sugar returns to normal.

Risk and Side Effects of the Procedure

The GTT procedure is generally safe. Some people may experience temporary side effects such as dizziness, mild nausea, diarrhea, or slight discomfort at the blood draw site, but these usually resolve quickly.

Gestational diabetes is a condition marked by elevated blood glucose levels during pregnancy. The standard diagnostic method is the 75-gram oral glucose tolerance test (OGTT).

One-Hour Glucose Challenge Test in Pregnancy

The initial screening involves consuming a 50 g glucose solution, followed by a blood test one hour later. If the glucose levels exceed the normal range for pregnancy, a confirmatory oral glucose tolerance test (OGTT) is advised.

Three-Hour OGTT Test in Pregnancy

The diagnostic OGTT involves consuming 100 g of glucose, followed by blood tests taken at multiple intervals over three hours.

Two-Hour 75 gm Glucose Test in Pregnancy

In India, the widely used test involves drinking 75 g of glucose, with blood sugar measured at fasting, 1 hour, and 2 hours after intake. It is simple, reliable, and forms the basis of most diagnostic guidelines.

A postprandial glucose test measures blood sugar 2 hours after a meal, showing how the body handles glucose in daily life. While useful for assessing meal-related spikes in diabetes or insulin resistance, it does not replace the more controlled OGTT.

Knowing the normal ranges is key to interpreting glucose tolerance test results.

Normal Glucose Levels for Type 2 Diabetes & Prediabetes

• Fasting: Less than 100 mg/dL = normal; 100-125 mg/dL = prediabetes; 126 mg/dL or above = diabetes.
• Two-hour post-OGTT: Less than 140 mg/dL = normal; 140-199 mg/dL = prediabetes; 200 mg/dL or above = diabetes.

Normal Glucose Levels During Pregnancy Chart

• Fasting: less than 95 mg/dL
• 1 hour after 75 gm glucose test: less than 180 mg/dL
• 2 hours after glucose 75 gm: less than 153 mg/dL

Q. What is the procedure of a glucose tolerance test?

A. A glucose tolerance test measures how your body processes sugar. Blood samples are taken before and after drinking a glucose solution to evaluate insulin function and detect diabetes.

Q. How do I prepare for a glucose tolerance test?

A. You should fast for 8–12 hours before the test, avoid heavy exercise, and inform your doctor about medications. Follow pre-test dietary guidelines for accurate results.

Q. What is the difference between the GTT and the postprandial glucose test?

A. The GTT measures glucose response after a fixed glucose load, while postprandial checks sugar levels two hours after a normal meal. Both help evaluate carbohydrate metabolism.

Q. What is the blood glucose postprandial range?

A. Normal postprandial blood glucose is usually under 140 mg/dL at two hours. Levels between 140–199 mg/dL indicate prediabetes, while 200 mg/dL or higher may suggest diabetes.

Q. What does the 75 g glucose test mean in pregnancy?

A. The 75-gram glucose test in pregnancy screens for gestational diabetes. It measures blood sugar before and two hours after consuming glucose, helping ensure maternal and fetal health.

Q. What is the glucose challenge test normal range in pregnancy?

A. In pregnancy, a 1-hour glucose challenge test result below 140 mg/dL is considered normal. Higher values may require a full glucose tolerance test for confirmation.

Q. What are the normal glucose levels during pregnancy?

A. Normal ranges: fasting <95 mg/dL, 1-hour <140 mg/dL, and 2-hour <120 mg/dL—key for monitoring maternal and fetal health.

Q. What is the difference between a 2-hour and a 3-hour glucose tolerance test?

A. A 2-hour test involves fasting, drinking 75 grams of glucose, and two-hour blood checks. The 3-hour test uses 100 grams of glucose with multiple readings, often for gestational diabetes.

Q. Can I drink water before the GTT test procedure?

A. Yes, plain water is generally allowed before and during a glucose tolerance test. It helps maintain hydration without affecting blood sugar readings or test results.

Q. How is the estimation of glucose done in a lab test?

A. In labs, glucose is measured using enzymatic methods such as glucose oxidase or hexokinase, with samples analysed chemically or via automated analysers for accuracy.