Glucose tolerance test
Encyclopedia
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
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.
and other organizations interested in diabetes agreed on a standard dose and duration.
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 tests.
(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.
"), and fasting levels repeatedly at or above 7.0 mmol/l (126 mg/dl) are diagnostic of diabetes.
The 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.
Impaired glucose tolerance is often associated with insulin resistance and is often seen in Polycystic Ovarian Syndrome.
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.
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT) or intravenous glucose challenge test (IVGCT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.
Medical test
A diagnostic test is any kind of medical test performed to aid in the diagnosis or detection of disease. For example:* to diagnose diseases, and preferably sub-classify it regarding, for example, severity and treatability...
in which glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
is given and blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
samples taken afterward to determine how quickly it is cleared from the blood. The test is usually used to test for diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
, insulin resistance
Insulin resistance
Insulin resistance is a physiological condition where the natural hormone insulin becomes less effective at lowering blood sugars. The resulting increase in blood glucose may raise levels outside the normal range and cause adverse health effects, depending on dietary conditions. Certain cell types...
, and sometimes reactive hypoglycemia
Reactive hypoglycemia
Reactive hypoglycemia, or postprandial hypoglycemia, is a medical term describing recurrent episodes of symptomatic hypoglycemia occurring within 4 hours after a high carbohydrate meal in people who do not have diabetes...
and acromegaly
Acromegaly
Acromegaly is a syndrome that results when the anterior pituitary gland produces excess growth hormone after epiphyseal plate closure at puberty...
, or rarer disorders of carbohydrate metabolism
Carbohydrate metabolism
Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms....
. 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.
Standard OGTT
Since the 1970s, the World Health OrganizationWorld Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...
and other organizations interested in diabetes agreed on a standard dose and duration.
Preparation
The patient is instructed not to restrict carbohydrateCarbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
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
Type I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
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
Fasting
Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...
(water is allowed) for 8–12 hours prior to the tests.
Procedure
- 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 glucoseGlucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
(blood sugarBlood sugarThe blood sugar concentration or blood glucose level is the amount of glucose present in the blood of a human or animal. Normally in mammals, the body maintains the blood glucose level at a reference range between about 3.6 and 5.8 mM , or 64.8 and 104.4 mg/dL...
), and sometimes insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. 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....
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. Sometimes other laboratory continues to collect blood for up to 3 hours depending on the requesting physician.
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 weightBody weightThe term body weight is used in daily English speech as well as in the contexts of biological and medical sciences to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some countries it is still measured in pounds or stones and pounds...
, 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 pregnancyPregnancyPregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
to screen for gestational diabetesGestational diabetesGestational diabetes is a condition in which women without previously diagnosed diabetes exhibit high blood glucose levels during pregnancy . It is widely accepted as a disease only in the United States, there is some question whether the condition is natural during pregnancy...
, with a screening test of 50 grams over one hour. If elevated, this is followed with a test of 100 grams over three hours.
Substances measured and variations
If renal glycosuriaRenal glycosuria
Renal 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 (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 glycaemiaImpaired fasting glycaemia
Impaired fasting glucose , more commonly known as pre-diabetes refers to a condition in which the fasting blood glucose level is consistently elevated above what is considered normal levels; however, it is not high enough to be diagnosed as diabetes mellitus...
"), and fasting levels repeatedly at or above 7.0 mmol/l (126 mg/dl) are diagnostic of diabetes.
The 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
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...
". Glucose levels above 11.1 mmol/l (200 mg/dl) at 2 hours confirms a diagnosis of diabetes.
| Glucose levels | | NORMAL | | impaired fasting glycaemia Impaired fasting glycaemia Impaired fasting glucose , more commonly known as pre-diabetes refers to a condition in which the fasting blood glucose level is consistently elevated above what is considered normal levels; however, it is not high enough to be diagnosed as diabetes mellitus... (IFG) | | impaired glucose tolerance 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 Mellitus Diabetes mellitus Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced... (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 |
Impaired glucose tolerance is often associated with insulin resistance and is often seen in Polycystic Ovarian Syndrome.
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.
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT) or intravenous glucose challenge test (IVGCT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.