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A '''glucose tolerance test''' in medical practice is the administration of [[glucose]] to determine how quickly it is cleared from the blood. The test is usually used to test for [[Diabetes mellitus|diabetes]], [[insulin resistance]], and sometimes [[reactive hypoglycemia]]. The glucose is most often given orally so the common test is technically an oral glucose tolerance test (OGTT). The test may be performed as part of a panel of tests, such as the [[comprehensive metabolic panel]].

==Preparation and cautions==

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 43 kg (94 lb), or exaggerated glucoses may produce a [[Type I and type II errors|false positive]] result.

==Procedure for OGTT==

The patient should have been [[fasting]] for the previous 8-14 hours (water is allowed).

Usually the OGTT is scheduled to begin in the morning (0700-0800) as glucose tolerance exhibits a diurnal rhythm with a significant decrease in the afternoon. A zero time (baseline) blood sample is drawn.

The patient is then given a glucose solution to drink. The standard dose since the late 1970s has been 1.75 grams of glucose per kilogram of [[body weight]], to a maximum dose of 75 g. It should be drunk within 5 minutes. Prior to 1975 a dose of 100 g was often used.

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. In research settings, samples may be taken on many different time schedules.

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.

==Interpretation of OGTT results==

Fasting plasma glucose should be below 6.1 mmol/l (110 mg/dl). Fasting levels between 6.1 and 7.0 mmol/l (110 and 126 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.

The 2 hour glucose level should be below 7.8 mmol/l (140 mg/dl). Levels between this and 11.1 mmol/l (200 mg/dl) indicate "[[impaired glucose tolerance]]." Glucose levels above 11.1 mmol/l (200 mg/dl) at 2 hours confirms a diagnosis of diabetes.

{| border=1 cellpadding=2 style="text-align:center"

|+ '''1999 WHO Diabetes criteria - Interpretation of Oral Glucose Tolerance Test'''

|-

! rowspan=1 | Glucose levels !! colspan=2 | NORMAL !! colspan=2 | [[impaired fasting glycaemia]] (IFG)!! colspan=2 | [[impaired glucose tolerance]] (IGT)!! colspan=2 | [[Diabetes Mellitus]] (DM)

|-

| align="center" |Venous Plasma || align="center" |Fasting || align="center" |2hrs || align="center" |Fasting || align="center" |2hrs || align="center" |Fasting || align="center" |2hrs || align="center" |Fasting || align="center" |2hrs

|-

| ''(mmol/l)'' || <6.1 || <7.8 || <u>></u> 6.1 & <7.0 || <7.8 || <7.0 || <u>></u>7.8 || <u>></u>7.0 || <u>></u>11.1

|-

| ''(mg/dl)'' || <110 || <140 || <u>></u>110 & <126 || <140 || <126 || <u>></u>140 || <u>></u>126 || <u>></u>200

|}