A glucose tolerance test (OGTT) measures how your body processes sugar and helps your doctor determine if you have or are at risk of diabetes. The test involves drinking a sugary solution, and then having your blood sugar levels measured at different intervals. If your body isn’t properly absorbing the sugar, it may indicate diabetes or prediabetes. The test can also detect gestational diabetes in pregnant women.

Why Do You Need a Glucose Tolerance Test?

Your doctor may recommend an OGTT if you:

• Are overweight or obese
• Have a family history of diabetes
• Have high blood pressure or high triglycerides
• Have polycystic ovary syndrome (PCOS)
• Delivered a baby over 9 pounds or had gestational diabetes during a previous pregnancy

The test is especially important for pregnant women between 24 and 28 weeks to check for gestational diabetes. If you have risk factors, you may need the test earlier in pregnancy or even a longer version of it.

Types of Glucose Tolerance Tests

There are different variations of the OGTT, including:

2-hour OGTT: Involves fasting and then measuring blood sugar levels 2 hours after drinking a sugary solution.  

3-hour OGTT: A more comprehensive test that involves fasting and measuring blood sugar levels at 1, 2, and 3 hours after drinking the glucose solution.

How to Prepare for the Test?

To prepare, eat normally in the days before your test. You will need to fast for 8 hours prior to the test, so it’s a good idea to schedule it in the morning. During the fasting period, you can only drink water. If you take medication, ask your doctor if it might affect your results.

What Happens During the Test?

The test usually takes place at your doctor’s office or a lab. First, your fasting blood sugar will be measured. Then, you'll drink a glucose solution. Blood will be drawn again at set intervals—1, 2, or 3 hours later—depending on whether the test is for type 2 or gestational diabetes.

For pregnant women at risk of gestational diabetes, the 3-hour test requires fasting, followed by drinking the glucose solution, and blood samples being drawn at 1-hour, 2-hour, and 3-hour intervals.

What Do the Results Mean?

A normal blood glucose level after the test is 139 mg/dL or lower.

If your result is between 140-199 mg/dL, you may have prediabetes.

Levels above 200 mg/dL indicate diabetes.

For pregnant women, the normal range is below 140 mg/dL after the 1-hour test.

If your results are elevated, your doctor may recommend further testing or lifestyle changes.

Risks and Side Effects

Side effects of the test are rare but may include nausea, bloating, or minor discomfort from the blood draw. If diagnosed with prediabetes or diabetes, lifestyle changes like diet and exercise can help manage your blood sugar levels.

By detecting diabetes early, the glucose tolerance test allows you and your doctor to take action and prevent complications. Early diagnosis and management can help you maintain good health and avoid the long-term consequences of diabetes.