Glucose Tolerance Test
Alternative names
Oral glucose tolerance test
Definition
The glucose tolerance test measures the body's ability
to metabolize glucose.
How the test is performed
Glucose is the sugar that the body uses for energy. Patients with diabetes
mellitus have high blood glucose levels. Glucose tolerance tests are one of the
tools for making the diagnosis of diabetes.
The most common glucose tolerance test is the oral glucose tolerance test (OGTT).
After an overnight fast, a patient drinks a solution containing a known amount
of glucose. Blood is obtained before the patient drinks the glucose solution,
and blood is drawn again every 30 to 60 minutes after the glucose is consumed
for up to 3 hours.
Blood glucose levels above normal limits at the times measured can diagnose
type 2 diabetes or gestational diabetes (high blood glucose during pregnancy).
Insulin levels may also be measured. (Insulin is the hormone produced by the
pancreas that moves glucose from the bloodstream into cells.)
The intravenous glucose tolerance test (IGTT) is not often used. In this
test, patients are given a known amount of glucose by vein for 3 minutes, and
blood insulin levels are measured before glucose and at 1 and 3 minutes. Insulin
levels below a standard threshold may predict the development of type 1 diabetes
in some patients.
How to prepare for the test
Fast for 12 hours and do not eat during the test. Consult your health care
provider if you are using medication that can interfere with the test results.
(See special considerations.)
Infants and children:
The physical and psychological preparation you can provide for this or any
test or procedure depends on your child's age, interests, previous experience,
and level of trust. For specific information regarding how you can prepare your
child, see the following topics as they correspond to your child's age:
- Infant test/procedure preparation (birth to 1 year)
- Toddler test/procedure preparation (1 to 3 years)
- Preschooler test/procedure preparation (3 to 6 years)
- Schoolage test/procedure preparation (6 to 12 years)
- Adolescent test/procedure preparation (12 to 18 years)
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain,
while others feel only a prick or stinging sensation. Afterward, there may be
some throbbing.
Why the test is performed
The oral glucose tolerance test is used to screen pregnant women for
gestational diabetes between 24 and 28 weeks of pregnancy. It may also be used
to diagnose diabetes in research studies and in cases where the disease is
suspected despite a normal fasting blood glucose.
Normal Values
For a 75-gram oral glucose tolerance test used to check for type 2 diabetes,
normal (nondiabetic) blood values are:
- fasting: 60 to 110 mg/dL
- 1 hour: less than 200 mg/dL
- 2 hours: less than 140 mg/dL. Between 140-200 mg/dL is considered impaired
glucose tolerance. This group is at increased risk for developing diabetes.
Greater than 200 mg/dL is diagnostic of diabetes mellitus
For a 50-gram oral glucose tolerance test used to screen for gestational
diabetes, normal blood values at 1 hour are less than 140 mg/dL.
For a 100-gram oral glucose tolerance test used to screen for gestational
diabetes, normal blood values are:
- fasting: less than 95 mg/dL
- 1 hour: less than 180 mg/dL
- 2 hour: less than 155 mg/dL
- 3 hour: less than 140 mg/dL
Note: mg/dL = milligrams per deciliter
What abnormal results mean
Greater than normal levels of glucose may indicate
- Diabetes
- Gestational diabetes
High glucose levels may be a manifestation of another clinical problem (for
example, Cushing's syndrome).
What the risks are
Risks from venipuncture include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
Veins and arteries vary in size from one patient to another and from one side
of the body to the other. Obtaining a blood sample from some people may be more
difficult than from others.
Special considerations
Interfering factors:
- Acute stress (for example, from surgery or an infection)
- Vigorous exercise
Several drugs may cause glucose intolerance, including the following:
-
Thiazide diuretics (e.g., hydrochlorothiazide)
-
Beta-blockers (e.g., propranolol)
-
Oral contraceptives
-
Corticosteroids (e.g., prednisone)
-
Some psychiatric medications
Before having the test, let your health care provider know if you are taking
any of these medications.
(From the National Institute of Health)
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