Oral GTT Test Procedure

In this Oral GTT test procedure post we have briefly explained about preparation of the patient, modern and classical GTT, abnormal GTT curve, factors affecting oral glucose tolerance test procedure.

Oral glucose tolerance test procedure (OGTT), also known as the glucose tolerance test, determines the body’s ability to metabolise and eliminate sugar (glucose) from the bloodstream. 

After fasting for a length of time, you must drink a syrupy solution. After that, a blood sample is taken to see if you’re properly metabolising glucose. Among other things, the oral glucose tolerance test procedure can be used to detect diabetes, gestational diabetes (diabetes during pregnancy), and prediabetes (high blood sugar that predicts type 2 diabetes). Both adults and children can safely do the oral glucose tolerance test procedure.

Patient has diabetes-like symptoms, but fasting blood sugar levels are equivocal (between 100 and 126 mg/dl). Excessive weight gain is observed during pregnancy among women who have previously had a large baby (weighing more than 4 kg) or a miscarriage. To eliminate the possibility of benign renal glucosuria.

There is no reason to perform an oral glucose tolerance test in someone who has been diagnosed with diabetes. Oral glucose tolerance test procedure has no bearing on diabetes monitoring. It should only be used for the initial diagnostic.

Oral GTT Test Procedure

The patient is advised to eat a high-carbohydrate diet for three days before the oral glucose tolerance test procedure. In addition, the evening before the test, a food comprising roughly 30-50 g of carbohydrate should be consumed. This is critical. Carbohydrates may not be tolerated in a normal person otherwise.

For at least two days prior to the test, the patient should refrain from taking any medications that may affect blood glucose levels. During the oral glucose tolerance test procedure, the patient should not smoke. It is best to avoid strenuous exertion the day before.

The patient should not eat anything after 8 p.m. the night before. Breakfast should be avoided. This is to ensure that you fast for 12 hours. Patients are encouraged to stay in the hospital for the two-hour waiting time without engaging in any physical activity. It is recommended that patients refrain from leaving the hospital and returning within two hours.

Modern and classical GTT

Oral Glucose Tolerance Test

Oral GTT Test Procedure: Oral glucose tolerance test procedure

Glucose load given is the same in both instances. In the classical procedure, the blood and urine samples are collected at ½ an hour intervals for the next 2½ hours. (Total six samples, including 0 hr sample). Glucose is estimated in all the blood samples. Urine samples are tested for glucose qualitatively. In modern times, only 2 blood samples are collected.

Abnormal GTT Curve

Impaired Glucose Tolerance: Impaired Glucose Regulation is another name for it (IGR). Here, plasma glucose levels are higher than normal but lower than diabetes levels. Fasting plasma glucose levels in IGT range from 110 to 126 mg/dl, while 2-hour post-glucose values range from 140 to 200 mg/dl.

Impaired Fasting Glycemia: Fasting plasma glucose is higher than normal (between 110 and 126 mg/dl), but 2 hour post-glucose is within normal limits (less than 140 mg/dl). These people do not require immediate treatment, but they must be monitored on a regular basis.

Gestational DM: When carbohydrate intolerance is discovered for the first time during a pregnancy, this term is used. This category does not include a known diabetic patient who becomes pregnant. Women with GDM are more likely to develop frank diabetes later in life. GDM is linked to an increased risk of neonatal mortality. Maternal hyperglycaemia stimulates the foetus to secrete more insulin, resulting in increased foetal growth and birth weight. Women should be re-evaluated after giving birth.

Alimentary Glucosuria: The fasting and 2 hr values are normal in this case, but there is an exaggerated rise in blood glucose following glucose ingestion. This is due to a faster rate of glucose absorption from the intestine. This is seen in patients who have had a gastrectomy or who have hyperthyroidism.

Renal Glucosuria: The normal renal glucose threshold is 175-180 mg/dl. If blood glucose levels rise above this level, glucose begins to appear in the urine. In general, elevated blood glucose levels are reflected in urine. However, when the renal threshold is lowered, glucose is excreted in the urine. Because SGLT2 is abnormal, renal tubular reabsorption is reduced. In these instances, blood glucose levels are within normal ranges. This is known as renal glycosuria. The renal threshold is physiologically reduced during pregnancy; this is a harmless condition that will not progress. In the third trimester of pregnancy, approximately 10% of pregnant women experience renal glucosuria. Renal glucosuria is linked to renal diseases characterised by renal tubular transport defects, such as Fanconi’s syndrome. Glucosuria is seen in these cases, as well as amino aciduria and phosphaturia. In some cases, the renal threshold may be raised when glucose does not appear in urine despite high blood glucose levels. GFR is reduced in this case with little or no impairment of tubular reabsorption. This can be seen in old age (arteriosclerosis) and in the Kimmelsteil-Wilson syndrome (diabetic nephrosclerosis).

Factors Affecting GTT

Oral glucose tolerance test procedure is used to detect insulin deficiency by measuring insulin levels. As a result, the patient is advised to follow a carbohydrate-rich diet for three days prior to the test. The patient is advised to rest during and before the test. Curve is elevated and prolonged in liver diseases. Cortisol is secreted during acute infections, so curve is elevated and prolonged. The curve will rise sharply in hyperthyroidism. However, Hypothyroidism is characterised by a flat curve.

Further Readings


  1. https://www.ncbi.nlm.nih.gov/books/NBK532915/
  2. https://emedicine.medscape.com/article/2049402-overview
  3. https://labtestsonline.org/tests/glucose-tolerance-test
  4. https://www.rima.org/web/bcmonline/18185.html
  5. https://www.diabetes.co.uk/oral-glucose-tolerance-test.html