Pathology Laboratory

Glucose Tolerance Test

Last updated on January 27th, 2023 at 08:59 pm

A glucose tolerance test measures how well your body’s cells can absorb glucose (sugar) after you consume a specific amount of sugar. Glucose tolerance test is also called as GTT. Doctors primarily use a glucose tolerance test to diagnose diabetes during pregnancy (called gestational diabetes). Gestational diabetes occurs when a pregnant woman who hasn’t had diabetes before pregnancy has high blood sugar levels because of the pregnancy

What is the glucose tolerance test?

GTT, short for the Glucose Tolerance Test, is a test designed to assess the body’s response to glucose. In GTT, the patient is given a glucose solution and blood samples are drawn afterword at intervals to measure how well the body cells can absorb glucose. There are several variations to the glucose tolerance test used in different conditions but, the most common one of them is the Oral glucose tolerance test or OGTT.

The OGTT is mainly used in the diagnosis of gestational diabetes. For OGTT, the patient is required to fast for 8-10 hours and then a fasting plasma glucose is tested after that oral glucose solution is given. After that, blood samples can be drawn up to 2-4 times at different intervals to measure the blood glucose. An OGTT is usually performed in the morning and as glucose levels usually fall by afternoon.

Why do a GTT test?

OGTT used to be the gold standard in the diagnosis of diabetes type 2 but, is now being replaced with other GTT methodologies. The GTT is primarily used for the diagnosis of diabetes, insulin resistance, impaired beta-cell function, carbohydrate metabolism disorder and also reactive hypoglycaemia and acromegaly.  

Who needs a glucose tolerance test?

The GTT is usually given to pregnant women during the 24th and 28th weeks of pregnancy. This test is also given to pregnant women who have diabetes symptoms or have the risk of developing diabetes prior to pregnancy.

Besides that, the GTT is also given to other patients who are experiencing symptoms of varied diseases that can cause high glucose levels in the bloodstream or restrict the proper absorption of glucose by the body cells.

What is the procedure for the glucose tolerance test?

  • Have a normal diet like any other day. 
  • Fasting is required for 8 to 10 hours prior to the test, and only water is allowed during this period.
  • At first, a zero -time or baseline blood sample is drawn.
  • Then the patient is given a specific dose of glucose solution to drink
  • After that, the blood samples are drawn at regular intervals to measure the blood sugar levels and also insulin levels in certain cases. The blood sampling can be done as requested by the doctor and could involve up to 6 hours of testing. 

Learn about 24 hour urine test

How is a glucose tolerance test performed?

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You will then be asked to drink a liquid containing a certain amount of glucose (usually 75 grams). Your blood will be taken again every 30 to 60 minutes after you drink the solution. The test may take up to 3 hours.

Oral glucose tolerance test for diabetes

A two-hour, 75-gram oral glucose tolerance test (OGTT) is used to test for diabetes. A laboratory technician will take a fasting lab draw of blood to test your fasting glucose level first. They’ll then ask you to drink 8 ounces of a syrupy glucose solution that contains 75 grams of sugar.

You’ll then wait in the laboratory for two hours. The technician will draw blood at the one- and two-hour marks.

Gestational diabetes testing

Your doctor may use two tests to help them determine if you have gestational diabetes. The first test uses the same two-hour test already described, and you’ll have a blood draw at both the one- and the two-hour mark.

The second test may involve two steps: first, a one-hour screening and then, a three-hour glucose tolerance test if the one-hour screening levels are elevated.

After blood draws to test fasting glucose, you’ll drink a solution with 50 grams of sugar. An hour later, you’ll give a blood sample. A lab technician will use this sample to measure your blood sugar level.

The second step of this test is only conducted if the first has a positive result. It involves a three-hour version of the OGTT.

In the three-hour test, a lab technician will ask you to consume a syrupy glucose solution that contains 100 grams of sugar. They’ll draw your blood when you’re fasting and at the one-, two-, and three-hour marks after you’ve drunk the glucose solution.

By taking several samples of your blood as your body processes the sugary drink, your doctor will tell how well your body can handle a sugar challenge.

Side effects of glucose tolerance test

There is a slight risk attached with Glucose tolerance test, which are as follows;

People with high levels of sugar have the following side effect in response to drinking the glucose solution, like;

  • Nausea
  • Stomach discomfort
  • Diarrhoea
  • Constipation

Risks attached to drawing out blood samples include the following;

  • Swelling and redness at the puncture site
  • Hematoma
  • Fever
  • Dizziness
  • Infection at the puncture site

What are the normal values of the GTT test?

The GTT normal value is lower than 140 mg/dL and if the blood glucose level is between 140 and 199 mg/dL, then it shows prediabetes. 

The OGTT normal range for fasting results is between 10 and 25 mg/dL for prediabetes, 126 mg/dL or greater for diabetes and greater than 92 mg/dL for gestational diabetes.

The OGTT normal range for after 2-hour test results is between 14 and 99 mg/dL for prediabetes, 200 mg/dL or greater for diabetes and greater than 153 mg/dL for gestational diabetes. 

After the glucose tolerance test,

For diabetes, your doctor may recommend that you take more tests before they make a diagnosis. No other testing will be done to diagnose gestational diabetes.

If your doctor diagnoses prediabetes or diabetes, they’ll recommend that you make diet and exercise changes. They may also prescribe diabetes medications as needed.

Doctors treat gestational diabetes with diet and activity, and your doctor will add medication to your treatment if you need it. Your doctor will ask you to monitor your blood sugar levels every day to make sure they’re within the recommended targets.

If you have gestational diabetes, you should start treatment right away.

Unmanaged diabetes can lead to having a larger-sized baby, which may cause complications during delivery, premature delivery, and other complications, like preeclampsia. Your doctor will work with you to develop a treatment plan tailored to your needs and health history.

You should also read about: Type 2 Diabetes

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