Glucose tolerance test

In patients who are thought to have abnormal blood glucose levels, glucose tolerance test is performed to determine their risky status and to investigate diabetes. Glucose tolerance testing is primarily done with an oral glucose test, although it is sometimes also done with intravenous glucose for research purposes. Since the glucose tolerance test with intravenous glucose has no clinical advantage over the oral test, the oral glucose tolerance test will be described here.

Abnormality in blood glucose levels is expressed as impaired fasting glucose. Abnormal glucose tolerance test and impaired fasting glucose are risk factors for type 2 diabetes. With the accompanying evaluations, it is evaluated whether the patient has diabetes mellitus or not. An oral glucose tolerance test may not be necessary if sufficient data are available for the diagnosis of diabetes.

For the diagnosis of diabetes mellitus:

• After an overnight fast, blood plasma glucose is above 126 mg/dl and this also occurs in repeated tests.
• The patient has symptoms of diabetes
• In the oral glucose tolerance test, after a 75 mg glucose load, the plasma glucose should be 200 mg/dl or higher.

Who is the oral glucose tolerance test applied to?

• People whose fasting blood glucose is between 110-126 mg/dL during the scans
• Pregnant women with suspected diabetes
• For demonstration of diabetes or glucose intolerance accompanying obesity
• Those with unexplained neuropathy, retinopathy, atherosclerosis, coronary artery disease or peripheral vascular disease at a young age
• To evaluate glucose metabolism after the acute state has passed in persons with hyperglycemia or glycosuria in stressful situations such as trauma, surgery, myocardial infarction.

How is the oral glucose tolerance test done?

The oral glucose tolerance test is applied to people who do not have complaints of acute or chronic diseases that cause carbohydrate intolerance. In addition, the patient should not have taken drugs that will increase blood glucose during the test. Before starting the oral glucose tolerance test, the patient should follow a diet containing at least 150 g of carbohydrates per day for three days.



The oral glucose tolerance test should be performed after at least 10 hours of overnight fasting. This fasting period should not exceed 16 hours. The patient undergoing an oral glucose tolerance test should sit and not smoke throughout the test.

A blood sample is taken for the patient’s fasting blood glucose. 1.75 mg/kg or 75 g glucose is given by measuring the patient’s weight. If gestational diabetes is being investigated in the patient, 100 g glucose is given. The patient drinks this glucose as syrup. As soon as the patient drinks the glucose syrup, the time is reset and the time is kept. Blood samples are taken at 30 minutes, 1 hour, 2 hours and 3 hours. An oral glucose tolerance test normally takes 3 hours. However, if the patient has a suspicion of reactive hypoglycemia, the test can be extended until the blood glucose rises.

What is an abnormal oral glucose tolerance test result?

In a healthy person, the blood glucose level in the blood sample taken at the 2nd hour is expected to be below 140 mg/dL. Measuring the glucose value between 140 and 200 mg/dL is defined as impaired glucose tolerance and these people are defined as those who should be followed up for the development of diabetes in the future. Diabetes is diagnosed when the 2nd hour glucose value is above 200 mg/dL.

How is the oral glucose tolerance test evaluated?

Some evaluations in the glucose tolerance test may indicate that the patient does not have diabetes but glucose intolerance. The interpretation of glucose intolerance can be made in the following cases:

• If the patient’s fasting glucose level is below the level required for the diagnosis of diabetes
• If the blood glucose level after the 2nd hour in the oral glucose tolerance test is between normal values and diabetic values
• If glucose levels in the oral glucose tolerance test between the 30th minute and the 2nd hour are higher than normal values

In order to diagnose diabetes by performing an oral glucose tolerance test in adults who are not pregnant, the following conditions must be determined:

• Diabetes symptoms such as drinking a lot of water (polyuria), peeing a lot (polydipsia), elevated blood glucose, ketones in the urine
• High fasting blood glucose level in several measurements
• If the fasting glucose level is below the diabetic values, high detection of blood glucose measurements up to the 2nd hour and at the 2nd hour in the oral glucose tolerance test.

Oral glucose tolerance test in pregnant women is performed in pregnant women who have not been found to have glucose intolerance before. Oral glucose tolerance test in pregnant women is done between the 24th and 28th weeks of pregnancy. 50 g glucose is taken orally at any time of the day without waiting for fasting in pregnant women. If the blood glucose level is equal to or higher than 140 mg/dl after one hour, it is decided to perform a classical glucose tolerance test. If the blood glucose is above normal values in two or more measurements in the classical glucose tolerance test in pregnant women, it is valuable for the diagnosis of gestational diabetes. Expected blood glucose limits in oral glucose tolerance test in pregnant women:

• Hunger 105 mg/dl
• 1st hour 190 mg/dl
• 2nd hour 165 mg/dl
• 3rd hour 145 mg/dl

An oral glucose tolerance test may show abnormal results in people over 60 years of age, even if they do not have diabetes mellitus. Or even if there is diabetes, the test results can be detected as normal. Since glucose tolerance is reduced by around 50% in the elderly, it may require the use of other accompanying symptoms and data together.

Causes of abnormal glucose tolerance test other than diabetes:

• Prepare with an improper diet
• Hepatocellular diseases
• Chronic diseases
• Prolonged physical inactivity
• Malnutrition and hunger
• Primary aldosteronism
• kidney diseases
• Alcoholism
• Celebral hemorrhage
• Cushing’s syndrome
• Stress due to febrile illness
• Acromegaly
• Adrenocortical hyperfunction
• Long-term steroid therapy
• Islet cell tumors
• Chronic kidney disease
• Uremia
• After gastric surgery
• Medicines