Glucose tolerance test
| Glucose tolerance test | |
|---|---|
 A diagram of glucose metabolism | |
| Purpose | To determine how quickly glucose is cleared from blood |
The glucose tolerance test (GTT, not to be confused with GGT 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.[1] The test is usually used to test for diabetes, insulin resistance, impaired beta cell function,[2] 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.[3] 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.
History
[edit]The glucose tolerance test was first described in 1923 by Jerome W. Conn.[4]
The test was based on the previous work in 1913 by A. T. B. Jacobson in determining that carbohydrate ingestion results in blood glucose fluctuations,[5] and the premise (named the Staub-Traugott Phenomenon after its first observers H. Staub in 1921 and K. Traugott in 1922) that a normal patient fed glucose will rapidly return to normal levels of blood glucose after an initial spike, and will see improved reaction to subsequent glucose feedings.[6][7]
Testing
[edit]Since the 1970s, the World Health Organization and other organizations interested in diabetes agreed on a standard dose and duration.[8]
Preparation
[edit]The patient is instructed not to restrict carbohydrate intake in the days or weeks before the test.[9] 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 42.6 kg (94 lb), or the excessive glucose 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. Medication such as large doses of salicylates, diuretics, anticonvulsants, and oral contraceptives affect the glucose tolerance test.[10]
Procedure
[edit]- A zero time (baseline) blood sample is drawn.
- The patient is then given a measured dose (below) of glucose solution to drink within a 5-minute time frame.
- Blood is drawn at intervals for measurement of glucose (blood sugar), and sometimes insulin levels. The intervals and number of samples vary according to the purpose of the test. For simple diabetes screening, the most important sample is the 2 hour sample and the 0 and 2 hour samples may be the only ones collected. A laboratory may continue to collect blood for up to 6 hours depending on the protocol requested by the physician.
Dose of glucose and variations
[edit]- 75 g of oral dose is the recommendation of the WHO to be used in all adults,[11] and is the main dosage used in the United States.[12] The dose is adjusted for weight only in children.[11] The dose should be drunk within 5 minutes.
- A variant is often used in pregnancy to screen for gestational diabetes, with a screening test of 50 g over one hour. If elevated, this is followed with a test of 100 g over three hours.