Glucose Tolerance Test (GTT)

Glucose Tolerance Test (GTT)- Why am I having this test?

The glucose tolerance test is done to check how your body processes sugar (glucose). This is one of several tests used to diagnose diabetes (diabetes mellitus).

Your health care provider may recommend this test if you:

• Have a family history of diabetes.
• Are very overweight (obese).
• Have infections that keep coming back (recurring).
• Have had a lot of wounds that did not heal quickly, especially on your legs and feet.
• Are a woman and have a history of giving birth to very large babies or a history of repeated fetal loss (stillbirth).
• Have had high glucose levels in your urine or blood:
  • During a past pregnancy.
  • After a heart attack, surgery, or prolonged periods of high stress.

What is being tested?

This test measures the amount of glucose in your blood at different times during a period of 2 hours. This indicates how well your body is able to process glucose.

What kind of sample is taken?

Blood samples are required for this test. They are usually collected by inserting a needle into a blood vessel.

How do I prepare for this test?

• For 3 days before your test, eat normally. Have plenty of carbohydrate-rich foods.
• Follow instructions from your health care provider about:
  • Eating or drinking restrictions on the day of the test. You may be asked to not eat or drink anything other than water (fast) starting 8–12 hours before the test.
  • Changing or stopping your regular medicines. Some medicines may interfere with this test.

Tell a health care provider about:

• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders you have.
• Any surgeries you have had.
• Any medical conditions you have.
• Whether you are pregnant or may be pregnant.

What happens during the test?

First, your blood glucose will be measured. This is referred to as your fasting blood glucose, since you fasted before the test. Then, you will drink a glucose solution that contains a certain amount of glucose. Your blood glucose will be measured again 1 and 2 hours after drinking the solution.

This test takes 2 hours to complete. You will need to stay at the testing location during this time. During the testing period:

• Do not eat or drink anything other than the glucose solution. You will be allowed to drink water.
• Do notexercise.
• Do notuse any products that contain nicotine or tobacco, such as cigarettes and e-cigarettes. If you need help stopping, ask your health care provider.

The testing procedure may vary among health care providers and hospitals.

How are the results reported?

Your results will be reported as milligrams of glucose per deciliter of blood (mg/dL) or millimoles per liter (mmol/L). Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

• Fasting: less than 110 mg/dL (6.1 mmol/L).
• 1 hour after drinking glucose: less than 180 mg/dL (10.0 mmol/L).
• 2 hours after drinking glucose: less than 140 mg/dL (7.8 mmol/L).

What do the results mean?

Results that are within the reference ranges are considered normal, meaning that your glucose levels are well controlled. Results higher than the reference ranges may mean that you recently experienced stress, such as from an injury or a sudden (acute) condition like a heart attack or stroke, or that you have:

• Diabetes.
• Cushing syndrome.
• Tumors such as pheochromocytoma or glucagonoma.
• Kidney failure.
• Pancreatitis.
• Hyperthyroidism.
• An infection.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• The glucose tolerance test (GTT) is done to check how your body processes sugar (glucose). This is one of several tests used to diagnose diabetes (diabetes mellitus).
• This test measures the amount of glucose in your blood at different times during a period of 2 hours. This indicates how well your body is able to process glucose.
• Talk with your health care provider about what your results mean.

Glucose Tolerance Test During Pregnancy

This is performed to diagnose diabetes that develops during pregnancy which is called gestational diabetes mellitus.

Gestational diabetes is a temporary form of diabetes that some women develop during pregnancy. It usually occurs during the second trimester of pregnancy and goes away after delivery.

Testing (screening) for gestational diabetes usually occurs between 24 and 28 weeks of pregnancy. You may have the GTT test after having a 1-hour glucose screening test if the results from that test indicate that you may have gestational diabetes. You may also have this test if:

• You have a history of gestational diabetes.
• You have a history of giving birth to very large babies or have experienced repeated fetal loss (stillbirth).
• You have signs and symptoms of diabetes, such as:
  • Changes in your vision.
  • Tingling or numbness in your hands or feet.
  • Changes in hunger, thirst, and urination that are not otherwise explained by your pregnancy.

What is being tested?

This test measures the amount of glucose in your blood at different times during a period of 3 hours. This indicates how well your body is able to process glucose.

What kind of sample is taken?

Blood samples are required for this test. They are usually collected by inserting a needle into a blood vessel.

How do I prepare for this test?

• For 3 days before your test, eat normally. Have plenty of carbohydrate-rich foods.
• Follow instructions from your health care provider about:
  • Eating or drinking restrictions on the day of the test. You may be asked to not eat or drink anything other than water (fast) starting 8–10 hours before the test.
  • Changing or stopping your regular medicines. Some medicines may interfere with this test.

Tell a health care provider about:

• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders you have.
• Any surgeries you have had.
• Any medical conditions you have.

What happens during the test?

First, your blood glucose will be measured. This is referred to as your fasting blood glucose, since you fasted before the test. Then, you will drink a glucose solution that contains a certain amount of glucose. Your blood glucose will be measured again 1, 2, and 3 hours after drinking the solution.

This test takes about 3 hours to complete. You will need to stay at the testing location during this time. During the testing period:

• Do noteat or drink anything other than the glucose solution.
• Do notexercise.
• Do notuse any products that contain nicotine or tobacco, such as cigarettes and e-cigarettes. If you need help stopping, ask your health care provider.

The testing procedure may vary among health care providers and hospitals.

How are the results reported?

Your results will be reported as milligrams of glucose per deciliter of blood (mg/dL) or millimoles per liter (mmol/L). Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

• Fasting: less than 95–105 mg/dL (5.3–5.8 mmol/L).
• 1 hour after drinking glucose: less than 180–190 mg/dL (10.0–10.5 mmol/L).
• 2 hours after drinking glucose: less than 155–165 mg/dL (8.6–9.2 mmol/L).
• 3 hours after drinking glucose: 140–145 mg/dL (7.8–8.1 mmol/L).

What do the results mean?

Results within reference ranges are considered normal, meaning that your glucose levels are well-controlled. If two or more of your blood glucose levels are high, you may be diagnosed with gestational diabetes. If only one level is high, your health care provider may suggest repeat testing or other tests to confirm a diagnosis.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• The glucose tolerance test (GTT) is one of several tests used to diagnose diabetes that develops during pregnancy (gestational diabetes mellitus). Gestational diabetes is a temporary form of diabetes that some women develop during pregnancy.
• You may have the GTT test after having a 1-hour glucose screening test if the results from that test indicate that you may have gestational diabetes. You may also have this test if you have any symptoms or risk factors for gestational diabetes.
• Talk with your health care provider about what your results mean.