An oral glucose tolerance test is a medical test for diabetes given to patients in which 75g glucose is ingested by mouth and blood samples taken both before and after two hours in order to determine how quickly it is cleared from the blood. By determining the body’s ability to metabolise the glucose, diabetes and its pre-cursors as well as a number of other conditions can be tested for. Since the early 1970s, the World Health Organisation has provided the standard protocol globally.
Research by the World Health Organisation suggests that the Fasting Plasma Glucose Test, another common test for diabetes, fails to identify around 30% of cases. Similarly, the OGTT is the only way to identify patients with Impaired Fasting Glycaemia and Impaired Glucose Tolerance, which allows for treatment to stem the onset of diabetes.
The patient is instructed not to restrict carbohydrate intake in the days or weeks before the test. The test should not be done during an illness, as results may not reflect the patient's glucose metabolism when healthy. A full adult dose should not be given to a person weighing less than 43kg (94lb), or exaggerated glucoses may produce a false positive result. Usually the OGTT is performed in the morning as glucose tolerance can exhibit a diurnal rhythm with a significant decrease in the afternoon. The patient is instructed to fast (water is allowed) for 8–12 hours prior to the tests.
Ensure that the patient has consumed a normal diet for the previous three days before the test and confirm that the patient has fasted at least nine hours prior to the test. The ideal test time is 9.00am.
After first blood collection, allow the patient to drink the full contents of the Rapilose™ OGTT Solution pouch over a five to 10 minute period. After two hours take a second blood sample. The patient should sit quietly during the test period and should not smoke. Sips of water are allowed.
The OGTT should be interpreted in accordance with the venous plasma glucose levels as specified by the World Health Organisation and the International Diabetes Federation set out in the table below.