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Glucose tolerance test (aka glucose challenge test) is a medical test in which glucose is given and blood samples taken afterward to determine how quickly it is cleared from the blood. The test is usually used to test for diabetes, insulin resistance, and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. In the most commonly performed version of the test, an oral glucose tolerance test (OGTT), a standard dose of glucose is ingested by mouth and blood levels are checked two hours later. Many variations of the GTT have been devised over the years for various purposes, with different standard doses of glucose, different routes of administration, different intervals and durations of sampling, and various substances measured in addition to blood glucose.
Since the 1970s, the World Health Organization and other organizations interested in diabetes agreed on a standard dose and duration.
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 42.6 kg (94 lb), 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
If renal glycosuria (sugar excreted in the urine despite normal levels in the blood) is suspected, urine samples may also be collected for testing along with the fasting and 2 hour blood tests.
Fasting plasma glucose (measured before the OGTT begins) should be below 6.1 mmol/L (110 mg/dL). Fasting levels between 6.1 and 7.0 mmol/L (110 and 125 mg/dL) are borderline ("impaired fasting glycaemia"), and fasting levels repeatedly at or above 7.0 mmol/L (126 mg/dL) are diagnostic of diabetes.
A 2 hour OGTT glucose level below 7.8 mmol/L (140 mg/dL) is normal, whereas higher glucose levels indicate hyperglycemia. Blood plasma glucose between 7.8 mmol/L (140 mg/dL) and 11.1 mmol/L (200 mg/dL) indicate "impaired glucose tolerance", and levels above 11.1 mmol/L (200 mg/dL) at 2 hours confirms a diagnosis of diabetes.
It is important to stress that the diagnosis criteria stated above by the World Health Organisation (WHO) is for venous samples only i.e. a blood sample taken from a vein in the arm. An increasingly popular method for measuring blood glucose is from a capillary or finger-prick sample. This is less invasive, more convenient for the patient and requires minimal training to conduct. Whilst fasting blood glucose have been shown to be similar in both capillary and venous samples, postprandial blood glucose levels (those measured after a meal) can vary. Therefore the diagnosis criteria issued by the WHO should only be used for venous blood samples. Given the increasing popularity of capillary testing the WHO has recommended that a conversion factor between the two sample types be calculated. To date no conversion factor has been issued by the WHO despite some medical professionals adopting their own.
A standard 2 hour OGTT is sufficient to diagnose or exclude all forms of diabetes mellitus at all but the earliest stages of development. Longer tests have been used for a variety of other purposes, such as detecting reactive hypoglycemia or defining subsets of hypothalamic obesity. Insulin levels are sometimes measured to detect insulin resistance or deficiency.
The OGTT is of limited value in the diagnosis of reactive hypoglycemia, since (1) normal levels do not preclude the diagnosis, (2) abnormal levels do not prove that the patient's other symptoms are related to a demonstrated atypical OGTT, and (3) many people without symptoms of reactive hypoglycemia may have the late low Glucose.
The oral glucose challenge test (OGCT) is a short version of the OGTT, used to check pregnant women for signs of gestational diabetes. It can be done at any time of day, not on an empty stomach. The test involves 50g of glucose, with a reading after one hour.