Oral Glucose Tolerance Test – GTT


Last Updated December 20th, 2021

What is Glucose Tolerance Test?

Glucose tolerance test (GTT) is a screening test performed in order to determine the response of your body to glucose. It is also known as the Oral glucose tolerance test (OGTT). The two organs intrinsically involved in the metabolism and homeostasis of glucose are the pancreas and liver. Before we move further, it is imperative that we go through a few important definitions.

Glucose– It is a carbohydrate belonging to the category of monosaccharides. In fact, it is the most abundant monosaccharide. It is a simple sugar and its name is derived from the ancient Greek word gleukos which means “sweet delightful wine.” Plants manufacture glucose during photosynthesis with the aid of energy from the sun. For a healthy adult, normal blood sugar levels are between 4.0 to 5.4 mmol/L when fasting and up to 7.8 mmol/ 2 hours after eating.

Insulin– It is a hormone secreted by the beta cells of the pancreas and is essential for the metabolism of glucose that is derived from foods that you eat. Insulin keeps the blood sugar levels within the normal range. This is known as endogenous insulin. Exogenous insulin is used therapeutically in patients suffering from diabetes.

Diabetes– Diabetes is a disease in which your body either doesn’t make insulin or your body’s ability to utilize insulin is impaired. The result is same-increased blood sugar levels. Diabetes can be broadly classified into the following types-

Type 1 diabetes also known as insulin-dependent diabetes mellitus (IDDM). In this case, the beta cells of the pancreas which produce insulin are destroyed due to autoimmune irregularity. This results in a deficiency of insulin in the body.

Type 2 diabetes-also known as non-insulin dependent diabetes mellitus (NIDDM). In this scenario, the cells of your body become resistant to the effects of insulin. It’s the most common type of diabetes. It was earlier known as adult-onset diabetes, but its incidence has been on the rise in children of late; the primary reason is a surge in childhood obesity.

Gestational diabetes mellitus (GDM)-It is a type of diabetes that manifests itself during the second or third trimester of pregnancy.
Diabetes is secondary to other underlying conditions such as cystic fibrosis, pancreatitis, drug-induced diabetes (eg. steroid-induced diabetes), monogenic diabetes syndromes, and following organ transplantation.

What are the indications for the Glucose Tolerance Test?

Glucose tolerance test is indicated when it is difficult to establish or rule out diabetes mellitus with the help of random/fasting blood glucose testing. Glucose tolerance test is a gold standard in testing for diabetes when the results of random/fasting blood glucose testing are ambiguous (fasting 5.5–6.9; random 5.5–11.0 mmol/L).

A glucose tolerance test should not be performed in cases where fasting or random blood glucose values are clearly in the nondiabetic or diabetic range. (fasting or random <5.5; fasting ≥7.0 or random ≥11.1 mmol/L respectively).

All pregnant women should undergo an oral glucose tolerance test to rule out gestational diabetes at 24-28 weeks of gestation. The timing is important because insensitivity to insulin increases during the second trimester and glucose levels rise in women who do not have the ability to produce enough insulin to accept this resistance. It is important to diagnose and correct gestational diabetes because hyperglycemia in pregnancy is associated with an adverse prognosis both for the mother and the child.

Glucose tolerance test is indicated in non-pregnant women with the polycystic ovarian disease (PCOD) to ascertain their glycemic status. Glucose tolerance test is also indicated to screen for insulin resistance, impaired beta-cell function, reactive hypoglycemia, acromegaly, and other disorders of carbohydrate metabolism.

How common is gestational diabetes in India?

As per the recent statistics, more than 4 million pregnant women in India have been diagnosed with gestational diabetes. The prevalence greatly varies, from 3.8% to 41%, within different geographical regions and socioeconomic levels.

How is the Oral Glucose Tolerance test (OGTT) performed?

– Preparation for oral glucose tolerance test begins 3 days prior to the stipulated date. You will be advised to consume a diet containing at least 150 gm of carbohydrates for at least 3 days. You will be advised to fast midnight onwards on the night before the test.

– It is advisable to discuss with your physician if you have a medical history and if you are consuming any medicine.

– A baseline glucose level is established with the help of a fasting sample. Blood is withdrawn with the help of either phlebotomy or intravenous access. You will then be asked to consume a glucose-rich syrupy drink which contains 75 gm of glucose. In pediatric patients, the dose is 1.75 g/kg of body weight, with the maximum dose being 75 grams.

– After consuming glucose, you will be instructed to continue fasting. You will also be advised to remain seated and not to undergo any activity. Drinking excess water is also dissuaded as it may hamper the results of the test. Blood sugar levels are then measured at regular intervals- the standard being 30 minutes, 1 hour and 2 hours.

– If you are being tested for type 2 diabetes, the results (in mmol/l) and their interpretation is as follows-

Category Fasting 2 hours
No diabetes
≤6 <7.8
Prediabetes 6.1–6.9 7.8–11.0
Diabetes ≥7.0 ≥11.1

What are the normal readings in the case of pregnant women?

In the case of screening for gestational diabetes, the interpretation is different. Moreover, there is no overnight fasting prescribed to a pregnant woman. According to the American Diabetes Association (ADA), low-risk women can be excluded from a glucose tolerance test. But as in India, women have an 11-fold risk of developing gestational diabetes mellitus, it is advocated that glucose tolerance testing be conducted universally among them.

According to the Diabetes in Pregnancy Study Group India (DIPSI) Criteria for diagnosis of GDM (75 gm OGTT), following are the norms-

Glucose Level (2 hours reading)Category
≥ 11.1 mmol/l Diabetes Mellitus
≥ 7.8 mmol/l GDM (gestational diabetes)
≥ 6.7 mmol/lDGGT (decreased gestational glucose tolerance)

What steps should be taken if one is diagnosed with gestational diabetes?

Apart from DIPSI, some other standards used for screening gestational diabetes mellitus are the WHO (World Health Organization), International Association of Diabetes and Pregnancy Study Group (IADPSG), and the Canadian Diabetes Association.

If you are diagnosed with gestational diabetes mellitus, your physician will instruct you on how to manage your blood glucose levels during pregnancy. Glucose tolerance test is repeated at 6–8 weeks and 12 weeks after delivery. If diagnosed with gestational diabetes (it usually resolves post-pregnancy), you run the risk of developing full-blown diabetes mellitus later in life.

What is the post-procedure care for OGTT ?

After the test, you are free to resume routine activities. No specific instructions regarding diet are imparted.

What is the price of a Oral Glucose Tolerance Test in India?

The cost of GTT in India lies between Rs 55 to Rs 60, depending upon the city and lab facilities available.

Are there any complications associated with this test?

There are no serious complications of oral glucose tolerance test. It is a minimally invasive procedure. Rare instances of bruising, bleeding and subsequent infection at the site from where blood is withdrawn have been reported. Slight dizziness may be encountered.

What are the advantages of an oral glucose tolerance test?

– The glucose tolerance test is easily reproducible.
– It employs simple calculations.
– It can recognize deranged post-prandial metabolism.
– It succeeds where random/fasting blood glucose tests fail.

What are the limitations of the Glucose Tolerance Test?

– The Glucose tolerance test does not distinguish between type 1 and type 2 diabetes.

– It does not predict your body’s response to hypoglycemic therapy.

– It does not throw light on the probability of developing complications arising in the future due to the chronic nature of diabetes.

– The results may vary depending upon your compliance leading up to the day of the test and the way the test is performed.

Are the glucose tolerance test (OGTT or GTT) and HB1Ac test the same?

No, these are different blood tests. Glycosylated hemoglobin (HbA1c) is recommended by The American Diabetes Association to diagnose type 2 diabetes mellitus. It has the advantage of providing an idea regarding the glycemic status of a person over the previous 3-4 months.

According to a recent study, elevated 30minPG (plasma glucose) is associated with a high risk of incident diabetes, even in individuals classified as NGT (normal glucose tolerance) by a traditional OGTT.

How prevalent is diabetes in India?

According to the Diabetes Federation of India (DFI)- “In India, about 50.9 million people suffer from diabetes, and this figure is likely to go up to 80 million by 2025, making it the ‘Diabetes Capital’ of the world. Delhi alone has an estimated 29.8 lakh people suffering from this disease (Indo-US collaborative study)”.

One in eight Indians has or is at risk of developing diabetes. Diabetes is a debilitating chronic disease with both macrovascular and microvascular complications. It affects most of the organs of the body.

A timely diagnosis of this silent killer can go a long way in combating the pandemic of diabetes. Oral glucose tolerance test is still worth its weight in gold as far as screening of diabetes mellitus is concerned.

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