A
glucose tolerance test in medical practice is the administration of
glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
to determine how quickly it is cleared from the blood. The test is usually used to test for
diabetesDiabetes mellitus —often referred to simply as diabetes—is a condition in which the body either does not produce enough, or does not properly respond to, insulin, a hormone produced in the pancreas. Insulin enables cells to absorb glucose in order to turn it into energy...
,
insulin resistanceInsulin resistance is the condition in which normal amounts of insulin are inadequate to produce a normal insulin response from fat, muscle and liver cells. Insulin resistance in fat cells reduces the effects of insulin and results in elevated hydrolysis of stored triglycerides in the absence of...
, and sometimes
reactive hypoglycemiaReactive hypoglycemia is a medical term describing recurrent episodes of symptomatic hypoglycemia occurring 2–4 hours after a high carbohydrate meal...
. 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 panelThe comprehensive metabolic panel, or chemical screen, is a standard suite of 14 blood tests which serves as an initial broad screening tool for physicians. Because it is often ordered as a routine part of an annual physical examination or check up, over time the CMP provides an important...
.
The patient is instructed not to restrict
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
intake in the days or weeks before the test.
A
glucose tolerance test in medical practice is the administration of
glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
to determine how quickly it is cleared from the blood. The test is usually used to test for
diabetesDiabetes mellitus —often referred to simply as diabetes—is a condition in which the body either does not produce enough, or does not properly respond to, insulin, a hormone produced in the pancreas. Insulin enables cells to absorb glucose in order to turn it into energy...
,
insulin resistanceInsulin resistance is the condition in which normal amounts of insulin are inadequate to produce a normal insulin response from fat, muscle and liver cells. Insulin resistance in fat cells reduces the effects of insulin and results in elevated hydrolysis of stored triglycerides in the absence of...
, and sometimes
reactive hypoglycemiaReactive hypoglycemia is a medical term describing recurrent episodes of symptomatic hypoglycemia occurring 2–4 hours after a high carbohydrate meal...
. 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 panelThe comprehensive metabolic panel, or chemical screen, is a standard suite of 14 blood tests which serves as an initial broad screening tool for physicians. Because it is often ordered as a routine part of an annual physical examination or check up, over time the CMP provides an important...
.
Preparation and cautions
The patient is instructed not to restrict
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
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 positiveIn statistics, the terms type I error and type II error are used to describe possible errors made in a statistical decision process...
result.
Procedure for OGTT
The patient should have been
fastingFasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. A fast may be total or partial concerning that from which one fasts, and may be prolonged or intermittent as to the period of 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 weightAlthough some people prefer the less-ambiguous term body mass, the term body weight is overwhelmingly used in daily English speech and in biological and medical science contexts to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some...
, 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
glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
(
blood sugarBlood sugar concentration, or glucose level, refers to the amount of glucose present in the blood of a human or animal. Normally, in mammals the blood glucose level is maintained at a reference range between about 3.6 and 5.8 mM...
), and sometimes
insulinInsulin is a hormone that has extensive effects on metabolism and other body functions, such as vascular compliance. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, and stopping use of fat as an energy...
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 glycosuriaRenal glycosuria, also known as renal glucosuria, is a rare condition in which the simple sugar glucose is excreted in the urine despite normal or low blood glucose levels. With normal kidney function, glucose is excreted in the urine only when there are abnormally elevated levels of glucose in...
(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 125 mg/dl) are borderline ("
impaired fasting glycaemiaImpaired fasting glycaemia or impaired fasting glucose refers to a condition in which the fasting blood glucose is elevated above what is considered normal levels but is not high enough to be classified as diabetes mellitus...
"), 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 toleranceImpaired Glucose Tolerance is a pre-diabetic state of dysglycemia, that is associated with insulin resistance and increased risk of cardiovascular pathology. IGT may precede type 2 diabetes mellitus by many years...
." Glucose levels above 11.1 mmol/l (200 mg/dl) at 2 hours confirms a diagnosis of diabetes.
1999 WHO Diabetes criteria - Interpretation of Oral Glucose Tolerance Test
| Glucose levels | | NORMAL | | impaired fasting glycaemia Impaired fasting glycaemia or impaired fasting glucose refers to a condition in which the fasting blood glucose is elevated above what is considered normal levels but is not high enough to be classified as diabetes mellitus...
(IFG) | | impaired glucose tolerance Impaired Glucose Tolerance is a pre-diabetic state of dysglycemia, that is associated with insulin resistance and increased risk of cardiovascular pathology. IGT may precede type 2 diabetes mellitus by many years...
(IGT) | | Diabetes MellitusDiabetes mellitus —often referred to simply as diabetes—is a condition in which the body either does not produce enough, or does not properly respond to, insulin, a hormone produced in the pancreas. Insulin enables cells to absorb glucose in order to turn it into energy...
(DM) |
|---|
| Venous Plasma |
Fasting |
2hrs |
Fasting |
2hrs |
Fasting |
2hrs |
Fasting |
2hrs |
| (mmol/l) |
<6.1 |
<7.8 |
> 6.1 & <7.0 |
<7.8 |
<7.0 |
>7.8 |
>7.0 |
>11.1 |
| (mg/dl) |
<110 |
<140 |
>110 & <126 |
<140 |
<126 |
>140 |
>126 |
>200 |
Variations
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 glucoses that are said to be characteristic. Using a glucose tolerance in this context resembles use of a
Rorschach testThe Rorschach test is a psychological test in which subjects' perceptions of inkblots are recorded and then analyzed using psychological interpretation, complex scientifically derived algorithms, or both...
in that it is often used to support a diagnosis that the patient and doctor are already reaching agreement on based on other evidence, but it is inadequate by itself to confirm or refute the diagnosis (unlike its use for diabetes).
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.
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