An oral glucose tolerance test, often also called a glucose challenge test, is a process in which glucose is administered orally and monitored to see how quickly the body is able to absorb it and clear it from the blood. This test is used to measure insulin resistance and diabetes.
Though often interchangeably used with glucose challenge test, the two are not the same. While in oral glucose tolerance test, pregnancy is not considered, the oral glucose challenge test is specifically carried out for determining gestation diabetes during pregnancy. Glucose tolerance test is also used to determine reactive hyperglycemia.
The oral glucose tolerance test is usually performed as a part of a panel of different tests like the comprehensive metabolism.
To start with the test, the patient is asked to fast for at least 8-14 hours. If you are ill, the test should not be taken before you get well again. Also, if you have been taking any long term medication, you should discuss the use with your doctor because certain medicines, especially corticosteroids can affect the test results.
Since glucose tolerance has a diurnal rhythm and decreases significantly in the afternoon, the test is taken early in the morning. To begin with, you will be given a glassful of glucose syrup which should be consumed quickly. The dose of glucose administered is usually 1.75 grams of glucose per kilogram of the body weight. Earlier, the maximum dose was about 100 grams. However, today the maximum dose is 75 g of glucose. Read more on GTT test.
For the measurement of glucose, blood is drawn at regular intervals. The glucose level is checked at regular intervals to monitor the rate of absorption and detect any anomalies. The numbers of samples that are taken are different, depending on the purpose of the test. If it is simple diabetes, the samples are taken over a period of two hours.
When the test is being conducted for a condition like renal glycosuria where even though blood sugar is normal, glucose is excreted into urine, samples of urine may also be collected. Along with this, two blood samples are collected over a period of two hours.
Normal glucose tolerance test results are below 6.1 millimole per liter or 110 milligram per deciliter. The fasting levels of blood glucose that range from 6.1 to 7.0 millimole per liter or 110 to 125 milligrams per deciliter border on abnormal and show impaired fasting glycaemia.