What is a glucose tolerance test

Glucose Tolerance Test also referred to as the Oral Glucose Tolerance Test (OGTT), is a method which can help to diagnose instances of diabetes mellitus or insulin resistance. Glucose tolerance test is a more substantial indicator of diabetes than finger prick testing.

The Oral Glucose Tolerance Test (OGTT) is a two-hour test that checks your blood glucose levels before and 2 hours after you drink a special sweet drink.

Glucose tolerance test or oral glucose tolerance test (OGTT) is used to determine whether your body has difficulty metabolizing intake of sugar/carbohydrate.

Oral glucose tolerance test can be a useful test in helping to diagnose:

• Pre-diabetes
• Gestational diabetes in pregnant women
• Insulin resistance
• Reactive hypoglycemia

The patient is asked to take a glucose drink and their blood glucose level is measured before and at intervals after the sugary drink is taken.

Why is oral glucose tolerance test performed?

Glucose is the sugar your body uses for energy. People with untreated diabetes have high blood glucose levels.

Most often, the first tests used to diagnose diabetes in people who are not pregnant are:

• Fasting blood glucose level: diabetes is diagnosed if it is higher than 126 mg/dL (7 mmol/L) on 2 different tests
• Hemoglobin A1c test: diabetes is diagnosed if the test result is 6.5% or higher

Glucose tolerance tests are also used to diagnose diabetes. The oral glucose tolerance test is used to screen for or diagnose diabetes in people with a fasting blood glucose level that is high, but is not high enough (above 125 mg/dL or 7 mmol/L) to meet the diagnosis for diabetes.

Abnormal glucose tolerance (blood sugar goes too high during the glucose challenge) is an earlier sign of diabetes than an abnormal fasting glucose.

Glucose tolerance test during pregnancy

A glucose tolerance test is a routine test during pregnancy that checks a pregnant woman’s blood glucose (sugar) level.

Gestational diabetes is high blood sugar (diabetes) that starts or is found during pregnancy.

Glucose tolerance test during pregnancy checks for gestational diabetes. Most pregnant women have a glucose screening test between 24 and 28 weeks of pregnancy. The test may be done earlier if you have a high glucose level in your urine during your routine prenatal visits, or if you have a high risk for diabetes.

Women who have a low risk for diabetes may not have the screening test. To be low-risk, all of these statements must be true:

• You have never had a test that showed your blood glucose was higher than normal.
• Your ethnic group has a low risk for diabetes.
• You DO NOT have any first-degree relatives (parent, sibling, or child) with diabetes.
• You are younger than 25 years old and have a normal weight.
• You have not had any bad outcomes during an earlier pregnancy.

For either the 3 hour glucose tolerance test (two-step test) or 2 hour glucose tolerance test (one-step test), eat your normal food in the days before your test. Ask your health care provider if any of the medicines you take can affect your test results.

3 hour glucose tolerance test

The 3 hour glucose tolerance test is a Two-Step Testing

During the first step, you will have a glucose screening test:

• You DO NOT need to prepare or change your diet in any way.
• You will be asked to drink a liquid that contains glucose.
• Your blood will be drawn 1 hour after you drink the glucose solution to check your blood glucose level.

If your blood glucose from the first step is too high, you will need to come back for a 3-hour glucose tolerance test. For this test:

• DO NOT eat or drink anything (other than sips of water) for 8 to 14 hours before your test. (You also cannot eat during the test.)
• You will be asked to drink a liquid that contains glucose, 100 grams (g) .
• You will have blood drawn before you drink the liquid, and again 3 more times every 60 minutes after you drink it. Each time, your blood glucose level will be checked.
• Allow at least 3 hours for this test.

2 hour glucose tolerance test

The 2 hour glucose tolerance test is a One-Step Testing

You need to go to the lab one time for a 2-hour glucose tolerance test. For this test:

• DO NOT eat or drink anything (other than sips of water) for 8 to 14 hours before your test. (You also cannot eat during the test.)
• You will be asked to drink a liquid that contains glucose (75 g).
• You will have blood drawn before you drink the liquid, and again 2 more times every 60 minutes after you drink it. Each time, your blood glucose level will be checked.
• Allow at least 2 hours for this test.

Glucose tolerance test results during pregnancy

Normal Results during pregnancy

The 3 hour glucose tolerance test (TWO-STEP TESTING)

Most of the time, a normal result for the glucose screening test is a blood sugar that is equal to or less than 140 mg/dL (7.8 mmol/L) 1 hour after drinking the glucose solution. A normal result means you DO NOT have gestational diabetes.

Note: mg/dL means milligrams per deciliter and mmol/L means millimoles per liter. These are two ways to indicate how much glucose is in the blood.

If your blood glucose is higher than 140 mg/dL (7.8 mmol/L), the next step is the oral glucose tolerance test. This test will show if you have gestational diabetes. Most women (about 2 out of 3) who take this test DO NOT have gestational diabetes.

The 2 hour glucose tolerance test (ONE-STEP TESTING)

If your glucose level is lower than the abnormal results described below, you do not have gestational diabetes.

Abnormal Results during pregnancy

The 3 hour glucose tolerance test (TWO-STEP TESTING)

Abnormal blood values for a 3-hour 100-gram oral glucose tolerance test are:

• Fasting: greater than 95 mg/dL (5.3 mmol/L)
• 1 hour: greater than 180 mg/dL (10.0 mmol/L)
• 2 hour: greater than 155 mg/dL (8.6 mmol/L)
• 3 hour: greater than 140 mg/dL (7.8 mmol/L)

The 2 hour glucose tolerance test (ONE-STEP TESTING)

Abnormal blood values for a 2-hour 75-gram oral glucose tolerance test are:

• Fasting: greater than 92 mg/dL (5.1 mmol/L)
• 1 hour: greater than 180 mg/dL (10.0 mmol/L)
• 2 hour: greater than 153 mg/dL (8.5 mmol/L)

If only one of your blood glucose results in the oral glucose tolerance test is higher than normal, your doctor may simply suggest you change some of the foods you eat. Then, your doctor may test you again after you have changed your diet.

If more than one of your blood glucose results is higher than normal, you have gestational diabetes.

Glucose tolerance test preparation

Before the oral glucose tolerance test you will be asked not to eat, or drink certain fluids, for up to 8 to 12 hours before the glucose tolerance test.

You may be asked to not take certain medications in the lead up to the oral glucose tolerance test, but only if these would affect the test results.

For the oral glucose tolerance test itself, you will first have blood taken to measure your blood glucose level before the test. The next stage is to take a very sweet tasting, glucose drink.

Further blood samples will then be taken either at regular intervals of say 30 or 60 minutes or a single test after 2 hours. The oral glucose tolerance test could take up to 3 hours.

Between blood tests you will need to wait so it’s best to have some reading material, or something else to keep you occupied, with you.

Glucose tolerance test results

People without diabetes

• Fasting value (before test): under 108 mg/dL (under 6 mmol/L)
• At 2 hours: under 140 mg/dL (under 7.8 mmol/L)

People with impaired glucose tolerance (prediabetes)

• Fasting value (before test): 108 to 126 mg/dL (6.0 to 7.0 mmol/L)
• At 2 hours: 140 to 199 mg/dL (7.8 to 11.0 mmol/L)

Diabetic levels

• Fasting value (before test): over 126 mg/dL (over 7.0 mmol/L)
• Diabetes is diagnosed at 2 hour blood glucose of greater than or equal to 200 mg/dL (over 11.0 mmol/L)

If you are within the impaired glucose tolerance (prediabetes) range, you will likely be advised to make lifestyle changes. In some cases, blood glucose lowering medication may be advised.

If you fall within the diabetic range, it is quite likely that blood glucose medication will be prescribed to help your body keep your blood glucose levels down.

What is Prediabetes?

Before people develop type 2 diabetes, they almost always have “prediabetes” — blood glucose levels that are higher than normal but not yet high enough to be diagnosed as diabetes.

Doctors sometimes refer to prediabetes as impaired glucose tolerance (IGT) or impaired fasting glucose (IFG), depending on what test was used when it was detected. Prediabetes puts you at a higher risk for developing type 2 diabetes and cardiovascular disease.

There are no clear symptoms of prediabetes, so, you may have it and not know it.

Some people with prediabetes may have some of the symptoms of diabetes or even problems from diabetes already. You usually find out that you have prediabetes when being tested for diabetes.

If you have prediabetes, you should be checked for type 2 diabetes every one to two years.

Results indicating prediabetes are:

• An HbA1C of 5.7% – 6.4%
• Fasting blood glucose of 100 – 125 mg/dl (5.6 – 6.9 mmol/L)
• An oral glucose tolerance test (OGTT) 2 hour blood glucose of 140 mg/dl – 199 mg/dl (7.8 – 11.0 mmol/L)

Preventing Type 2 Diabetes

You will not develop type 2 diabetes automatically if you have prediabetes. For some people with prediabetes, early treatment can actually return blood glucose levels to the normal range.

Research shows that you can lower your risk for type 2 diabetes by 58% by:

• Losing 7% of your body weight (or 15 pounds if you weigh 200 pounds)
• Exercising moderately (such as brisk walking) 30 minutes a day, five days a week

Don’t worry if you can’t get to your ideal body weight. Losing even 10 to 15 pounds can make a huge difference.

Glucose tolerance test side effects

Drinking the glucose solution is similar to drinking very sweet soda. Serious side effects from glucose tolerance test test are very uncommon.

Serious side effects from this test are very uncommon. With the blood test, some people feel nauseated, sweaty, lightheaded, or may even feel short of breath or faint after drinking the glucose. Tell your doctor if you have a history of these symptoms related to blood tests or medical procedures.

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Other risks associated with having blood drawn are slight, but may include:

• Excessive bleeding
• Multiple punctures to locate veins
• Fainting or feeling lightheaded
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)