Glucose Tolerance 3hr (75g)

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A glucose tolerance 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 or rarer diseases 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 than 43 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 test.

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.

Dose of glucose and variations
·In the US, dosing is by weight, and since the late 1970s has been 1.75 grams of glucose per kilogram of body weight, to a maximum dose of 75 g. Prior to 1975 a dose of 100 g was often used.  
·The WHO recommendation is for a 75g oral dose in all adults: the dose is adjusted for weight only in children.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 grams over one hour. If elevated, this is followed with a test of 100 grams over three hours.  

The OGTT should be done in the morning after an overnight fast of between 8 and 14 hours. During the three previous days the subject must have an unrestricted diet (containing at least 150 g carbohydrate per day) and unlimited physical activity. The subject should remain seated during the test and should not smoke throughout the test.
The test involves drinking a solution containing a certain amount of glucose, and drawing blood to measure glucose levels at the start and on set time intervals thereafter.
The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria lead to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.
The following are the values which the American Diabetes Association considers to be abnormal during the 100 g of glucose OGTT:
·Fasting blood glucose level =95 mg/dl (5.33 mmol/L)  
·1 hour blood glucose level =180 mg/dl (10 mmol/L)  
·2 hour blood glucose level =155 mg/dl (8.6 mmol/L)  
·3 hour blood glucose level =140 mg/dl (7.8 mmol/L)  
An alternative test uses a 75 g glucose load and measures the blood glucose levels before and after 1 and 2 hours, using the same reference values. This test will identify less women who are at risk, and there is only a weak concordance (agreement rate) between this test and a 3 hour 100 g test.
The glucose values used to detect gestational diabetes were first determined by O'Sullivan and Mahan (1964) in a retrospective cohort study (using a 100 grams of glucose OGTT) designed to detect risk of developing type 2 diabetes in the future. The values were set using whole blood and required two values reaching or exceeding the value to be positive. Subsequent information led to alterations in O'Sullivan's criteria. When methods for blood glucose determination changed from the use of whole blood to venous plasma samples, the criteria for GDM were also changed.

Test results
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.

2 hour OGTT 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.

WHO Diabetes criteria - Interpretation of Oral Glucose Tolerance Test

Glucose levels

impaired fasting glycaemia (IFG)

impaired glucose tolerance (IGT)

Diabetes Mellitus (DM)

Venous Plasma

> 6.1 & <7.0

>110 & <126

Impaired glucose tolerance is also called insulin resistance and is often seen in Polycystic Ovarian Syndrome.

Read more about Glucose.

All information on this page is intended for your general knowledge only and does not provide medical advice, diagnosis or treatment.