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GTT full form is Glucose Tolerance Test. It is a medical test. It checks how the body handles glucose. In this test, glucose is given by mouth. Blood samples are collected at fixed times. It shows how fast glucose clears from the blood.
The GTT is mostly used to detect diabetes. It also helps to find other health problems like insulin resistance, impaired beta cell function, reactive hypoglycemia, and rare disorders related to carbohydrate metabolism.
The most popular form of this test is the Oral Glucose Tolerance Test (OGTT). In this, the patient drinks a sugary liquid. After that, blood samples are taken at different times to check how the body handles the sugar.
Full Form of GTT
The Glucose Tolerance Test (GTT) is a common medical test. It helps doctors understand how well the body processes glucose (sugar). Glucose is the main source of energy for our bodies. After eating, our blood sugar level rises. The body controls it with the help of insulin. If this process doesn’t work properly, it may lead to diseases like diabetes.
Glucose Tolerance Test History
The Glucose Tolerance Test (GTT) was first explained in 1923. It was described by Jerome W. Conn. Before this, in 1913, A. T. B. Jacobson did related work. He showed that eating carbohydrates changes blood sugar levels. In 1921 and 1922, H. Staub and K. Traugott made more observations.
They found that in healthy people, blood sugar rises after glucose. But it quickly returns to normal levels. This effect was called the Staub-Traugott Phenomenon. They also noticed better sugar control with repeated glucose. These findings helped create the modern glucose tolerance test.
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GTT Testing
Since the 1970s, the World Health Organization (WHO) has given clear guidelines. Other health organizations also followed these rules. They agreed on the glucose dose for the test. They also fixed the time duration of the test. These standards help doctors check for diabetes in the same way everywhere.
Why Is the GTT Done?
Doctors suggest this test when they suspect problems with sugar metabolism. Common reasons for performing a GTT include:
- Diagnosing diabetes
- It is also used to check for insulin resistance.
- In order to identify gestational diabetes during pregnancy, GTT is crucial.
- It helps to study impaired glucose tolerance.
- Doctors use it for rare cases like reactive hypoglycemia.
- Researching disorders involving beta-cell dysfunction in the pancreas
Preparation Before the GTT Test
There are a few important things to remember before taking this test:
- The patient should not cut down on carbohydrates before the test.
- The test should not be done during any illness.
- Illness can affect the accuracy of glucose results.
- People below 42.6 kg should not be given a full adult dose.
- Extra glucose may give a false positive result.
- The OGTT is usually done in the morning.
- Glucose tolerance changes during the day.
- Eight to twelve hours prior to the test, you must fast.
- Drinking water is allowed during fasting.
- Some medicines can affect the test results.
- These include salicylates, diuretics, anticonvulsants, and oral contraceptives.
GTT Blood Sample Methods
The WHO guidelines are for venous blood samples only. Venous blood is taken from a vein in the arm. Another method is capillary blood sampling. This is done by a finger-prick. It is less painful & easier for the patient. It also needs less training to perform. Fasting glucose levels are similar in both methods. But after-meal (postprandial) glucose levels can differ.
WHO has not given rules for capillary testing yet. They suggest creating a conversion method. But no official conversion factor exists as of 2017. Some doctors have made their guidelines. A study in 2020 looked at pregnant women. It compared venous and capillary samples for gestational diabetes. Fasting results were almost the same in both methods.
But after 2 hours, the results were different. The study said capillary samples can still be useful. With proper adjustments, they work well during pregnancy. They help diagnose gestational diabetes accurately.
Glucose Tolerance Test Procedure
First, a baseline blood sample is taken. This is called the zero-time sample. Next, the patient drinks a glucose solution. The glucose must be consumed within 5 minutes. After this, blood samples are taken at set times. These samples measure blood sugar levels.
Sometimes insulin levels are also measured. The number of samples depends on the test purpose. For diabetes, usually 0 & 2-hour samples are enough. Some tests may last up to 6 hours. The doctor decides the exact timing of the samples.
How is the GTT Test Done?
The GTT is a common medical test. It helps medical professionals in assessing how your body processes sugar. The primary purpose of this test is to identify diabetes and associated conditions. The steps are as follows:
- First, a fasting blood sample is collected.
- You are given a sugary drink with 75 grams of glucose.
- You must finish this drink within 5 minutes.
- Blood samples are then taken at set times.
- For diabetes, samples are usually taken at 0 and 2 hours.
- Sometimes the test can go on for 6 hours.
- In some cases, urine samples are also taken.
- These help check for sugar in the urine.
- The doctor decides the exact process based on need.
Variations in the Glucose Dose
WHO recommends a 75-gram glucose dose for adults during the GTT. This is also the standard in the United States. For children, the dose depends on body weight. The drink must be finished within 5 minutes.
A 50-gram glucose test is often used by doctors first during pregnancy. A second test using 100 grams spread over three hours is conducted if the results are high. In the UK, doctors earlier used 394 ml of Lucozade for this test. Now, they prefer specially prepared glucose drinks.
Sometimes, urine samples are also taken. This is done if doctors suspect sugar is being passed into the urine even when blood sugar seems normal.
Normal GTT Results
The results of your glucose tolerance test show your blood sugar levels. The test measures the glucose level in milligrams per deciliter, or mg/dL. Each lab may use a slightly different normal range. Always read the reference range given in your report. These are the general ranges for a 2-hour glucose tolerance test:
- Normal: Below 140 mg/dL.
- Prediabetes: Between 140 and 199 mg/dL.
- Diabetes: 200 mg/dL or higher.
For pregnancy (gestational diabetes), the limits are different. Some GTT tests are done for other reasons. For example, acromegaly tests measure growth hormone levels.
What Happens if the GTT Results are Abnormal?
The doctor will give you advice if your results are abnormal. After a single abnormal GTT test, don’t worry too much. Diabetes cannot be confirmed by a single test. You might be asked to retake the test by your doctor. They might occasionally recommend an additional test, such as the A1C.
Variations of the GTT
A standard two-hour GTT is commonly used. It helps diagnose or rule out diabetes. It works well in almost all stages of diabetes. Only the earliest phases might go undetected. Longer versions of the test are also used. They help find issues like reactive hypoglycemia.
They can also study rare conditions like hypothalamic obesity. Sometimes insulin levels are checked during the test. This helps understand insulin resistance or deficiency. The Glucose Tolerance Test is not perfect for reactive hypoglycemia. Normal results do not rule out the problem. Abnormal results do not always explain symptoms. Some healthy people may show low glucose levels late in the test.
Oral Glucose Challenge Test (OGCT)
The OGCT test is simple. It can be done at any time of the day. There is no need to fast before the GTT test. The patient drinks a beverage containing fifty grams of glucose. An hour later, a blood sample is taken. This sample is used to measure glucose levels. This test helps doctors know if more tests are needed. It is often used to test for gestational diabetes.
Limitations of the GTT
The OGTT has some limits. It cannot clearly show the cause of the problem. It cannot tell if the issue is insulin resistance or low insulin production. It is less accurate than other tests. The hyperinsulinemic-euglycemic clamp is the gold standard. That test is more precise for checking insulin resistance. But OGTT is easier to do. It is used more often in clinics.
The insulin tolerance test is also more accurate. But both these tests are hard to use in regular clinics. They are mainly used in research, not daily medical practice. HOMA-IR is another method used to check insulin resistance. It is simple and useful in large studies. However, it may not give correct results in diabetic patients.
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FAQs on GTT Full Form
What is the glucose tolerance test, or GTT?
A medical test called the Glucose Tolerance Test (GTT) gauges how well your body breaks down sugar, or glucose.
What distinguishes GTT from OGTT?
The terms OGTT and GTT refer to the same test, which is the Oral Glucose Tolerance Test. The acronym GTT can be used to abbreviate the acronym OGTT, which stands for Oral Glucose Tolerance Test.
What is a normal Glucose Tolerance Test for a 2-hour 75g?
A glucose level below 7.8 mmol/L (140 mg/dL) is normal for a 2-hour GTT with a 75 g intake; higher levels signify hyperglycemia.
