|
Headlines:
|
 |
Back to Endocrinology Lab tests
Glucose Tolerance Test
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, insulin resistance, and
sometimes reactive hypoglycaemia. Most commonly the glucose is given
orally so the common test is technically an oral glucose tolerance
test (OGTT).
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.
Procedure for OGTT
The patient should have been fasting for the previous 8-14 hours.
A zero time (baseline) blood sample is drawn.
The patient is given a glucose solution to drink. The standard dose
since the late 1970's has been 1.75 grams of glucose per kg 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
the only ones collected.
Interpretation of OGTT results
Fasting glucose should be below 111 mg/dl. Fasting levels between 110
and 126 are borderline (“impaired fasting glucose”), and fasting
levels repeatedly above 125 are diagnostic of diabetes. The 2 hour
glucose should be below 140 mg/dl. Levels between 140 and 200 indicate
“impaired glucose tolerance.” Glucose above 200 at 2 hours confirms a
diagnosis of diabetes.
Whilst impaired glucose tolerance is an asymptomatic condition, it
carries an increased risk of large vessel disease.
In Europe, blood glucose is measured in mmol/l. 1 mmol/l=18 mg/dl. For
the values above, 110 mg/dl=6.2 mmol/l, and 125=7, 140=7.8, 200=11.1)
Variations
A standard 2 hour OGTT is sufficient to diagnose or exclude diabetes.
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 test 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.
|
|
|
|
Are you a doctor or a nurse?
Do you want to join the Doctors Lounge online medical community?
Participate in editorial activities (publish, peer review, edit) and
give a helping hand to the largest online community of patients.
Click on the link below to see the requirements:
Doctors Lounge Membership
Application |
|
|
send
to a friend
printer
friendly version
