Glucose Tolerance Test
Overview
The glucose tolerance test, also called the oral glucose tolerance test (OGTT), checks how the body handles glucose. Health professionals often use this tool to screen for type 2 diabetes, prediabetes, or issues with blood sugar even before symptoms appear. It also helps identify gestational diabetes during pregnancy.
A glucose tolerance test shows how the body processes sugar after eating. Doctors use it to detect high blood sugar levels and confirm a diagnosis, sometimes before diabetes or diabetes mellitus causes any noticeable symptoms.
Table: Possible Uses for the Glucose Tolerance Test
| Condition | Purpose |
|---|---|
| Type 2 Diabetes | Detects early signs or confirms diagnosis. |
| Prediabetes | Identifies at-risk individuals. |
| Gestational Diabetes | Monitors blood sugar changes during pregnancy. |
Guidelines from groups like the American Diabetes Association set the standards for using and interpreting this test as part of the broader process for classifying and diagnosing different forms of diabetes.
Possible Complications
Blood sample collection usually has minimal risks. Some people may notice mild bruising, slight bleeding, or feel dizzy. Rare cases involve infection at the needle site. Temporary side effects like low blood sugar (hypoglycemia) can also occur, especially during a glucose test.
Getting Ready for the Test
What to Eat and Which Medicines Matter
Continue to eat and drink as normal before the glucose tolerance test. Notify your care team if you feel sick or need to take medications, as this might change your blood sugar levels. Share information about usual activities, regular meals, and medication updates for the most accurate results.
Fasting Before the Test
Before starting the oral glucose tolerance test, you will usually need to fast for about eight hours. This means no food and no drinks, not even coffee or juice.
Many people find it helpful to schedule the test early in the morning and begin fasting the night before. This approach makes it easier to follow the fasting instructions and helps ensure that recent meals do not affect the test results.
What You Can Expect
Steps Taken at the Testing Site
Staff at a clinic, doctor’s office, hospital, or laboratory typically conduct the test. They begin by drawing blood to check the fasting blood glucose level. This first step provides a starting point for measuring changes in glucose levels as the test continues.
After the initial blood test, the specific steps change depending on the reason for the test—whether it’s to check for type 2 diabetes, prediabetes, or gestational diabetes. The steps below summarize what typically happens during a glucose tolerance test, along with how long each part usually takes:
| Step | What Happens | How Long It Takes |
|---|---|---|
| Fasting | No food/drink | 8 hours (usually overnight) |
| First Blood Draw | Before drink | A few minutes |
| Glucose Drink | 50, 75, or 100g sugar | 5 minutes |
| Next Blood Draw(s) | After 1, 2, or 3 hours | 1-3 hours total |
| Afterwards | Resume activities | Immediately (unless problems) |
Glucose Tolerance Testing for Type 2 Diabetes and Prediabetes
If a provider is checking for type 2 diabetes or prediabetes, you will drink a glucose solution that contains 75 grams of sugar. The taste is syrupy and quite sweet. After you drink the glucose, staff take blood samples at one hour and again at two hours.
Each blood draw checks your glucose levels to see how your body manages the sugar over time. This two-hour oral glucose tolerance test shows whether your body is processing sugar normally or if your blood sugar levels are staying too high.
Checking for Diabetes in Pregnancy
Providers usually test for gestational diabetes between weeks 24 and 28 of pregnancy, often during the third trimester. The testing process usually involves two steps:
Step 1: Glucose Challenge Test
- No fasting is needed for this part.
- Staff take a blood sample to check your baseline glucose level.
- You drink a glucose solution with 50 grams of sugar.
- One hour later, staff draw blood again to check how your body handles the sugar.
- If this result is high, you may need a second test.
Step 2: Three-Hour Glucose Tolerance Test
- Fasting for at least eight hours is required.
- Staff collect a fasting blood sample at the start.
- You drink a glucose solution with 100 grams of sugar.
- Blood samples are taken at one, two, and three hours after drinking it.
Some providers may use a one-step method. In this version, you fast, drink a 75-gram glucose solution, and have your blood tested at the start, one hour, and two hours later. The choice between tests depends on your risk factors and your provider’s guidance.
What Happens When the Test Is Over
After the test, most people can return to their normal routines, including eating and drinking as usual. Your provider may follow up to discuss the results and what they mean for your health.
If you notice any pain, swelling, changes in skin color, or other problems at the site where blood was drawn, contact your healthcare professional. These symptoms are uncommon but should be checked to make sure everything is healing properly.
Important Things To Remember
- The test may involve several blood draws over a few hours.
- You may need to stay in the clinic or lab between draws, especially for longer tests.
- Some people may feel sick to their stomach after drinking the glucose solution.
Test Outcomes
Results for Type 2 Diabetes and Prediabetes
The test results are usually measured two hours after drinking the glucose solution. Blood sugar levels are reported in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L), and help determine whether glucose is being processed normally.
Interpretation of Two-Hour Blood Sugar:
| Result Range | mg/dL | mmol/L | Meaning |
|---|---|---|---|
| Normal | Less than 140 | Less than 7.8 | Healthy glucose processing |
| In the warning zone | 140 – 199 | 7.8 – 11 | Indicates prediabetes |
| High | 200 or higher | 11.1 or higher | Often signals diabetes |
A value in the normal range means the body processes sugar as expected. Values in the middle range suggest impaired glucose tolerance, also called prediabetes. Prediabetes raises the risk for developing type 2 diabetes and heart issues. If the level reaches or goes above 200 mg/dL, it usually signals type 2 diabetes.
If results show possible diabetes or prediabetes, the healthcare provider might repeat the test another day or order a different blood test for confirmation. Some outside factors―such as illness, certain medicines, or level of physical activity―can affect these results and should be considered before making a diagnosis.
Findings for Pregnancy-Related Diabetes
For gestational diabetes, results are based on one or more timed blood sugar measurements. These may come from a one-hour screening or a follow-up three-hour glucose tolerance test, depending on the situation.
One-Hour Test Reference Ranges
- Below 140 mg/dL (7.8 mmol/L): Usually seen as within standard limits.
- 190 mg/dL (10.6 mmol/L) or more: Indicates gestational diabetes is likely.
If the one-hour result is higher than normal, a three-hour test is usually suggested.
Three-Hour Test Expected Values
| Time After Glucose Drink | Target Blood Sugar (mg/dL) | Target Blood Sugar (mmol/L) |
|---|---|---|
| Fasting | 95 or lower | 5.3 or lower |
| 1 hour | 180 or lower | 10.0 or lower |
| 2 hours | 155 or lower | 8.6 or lower |
| 3 hours | 140 or lower | 7.8 or lower |
If only one test result is too high, recommendations may include dietary changes, regular follow-up testing, and advice on healthy weight. If two or more results are higher than they should be, the diagnosis is usually gestational diabetes.
Some clinics use a two-hour version of the glucose tolerance test during pregnancy. In this approach:
- Fasting blood sugar at or above 92 mg/dL (5.1 mmol/L), or
- One-hour result of 180 mg/dL (10 mmol/L) or higher, or
- Two-hour result of 153 mg/dL (8.5 mmol/L) or above
Each of these values may indicate a diagnosis of gestational diabetes.
What Happens Next: If diagnosed, you may work with a nutrition specialist, and regular monitoring will be recommended. Managing blood sugar levels during pregnancy is important to lower the chance of health problems for both mother and baby.
