mellitus is a disorder characterized by abnormally
high blood glucose levels (hyperglycemia). Glucose,
a type of sugar, is the body's primary source of
energy. In type 1 diabetes, blood glucose is elevated
because the pancreas produces little or no insulin,
the hormone that permits cells to remove glucose
from the blood. In type 2 diabetes, high blood
glucose develops because the body's cells are resistant
The following glucose tests are performed
to diagnose or monitor diabetes:
single blood sample taken anytime after eating
that day may be sufficient for a diagnosis. A blood
glucose level above 200 milligrams per deciliter
(mg/dL) associated with the classic symptoms of
hyperglycemia (thirst, frequent urination, and
weight loss) indicates that diabetes is present.
fasting blood glucose test measures blood glucose
levels after a 12- to 14-hour fast. While levels
normally decrease during fasting, they remain persistently
high in people with diabetes. A fasting glucose
value above 125 mg/dL on at least two tests indicates
postprandial blood glucose test measures blood
glucose levels two hours after eating a meal. This
test is usually done in people who have symptoms
of hyperglycemia, or when the results of a fasting
glucose test suggest possible diabetes but are
inconclusive. Values of 200 mg/dL or more indicate
oral glucose tolerance test is not necessary in
most cases but is the method of choice to detect
diabetes when results from the fasting and postprandial
tests are borderline or inconclusive. In this test,
glucose levels in the blood and urine are measured
periodically for several hours following the ingestion
of a beverage containing a specified dose (usually
75 grams) of glucose.
A1c (HbA1c), also known as the glycosylated hemoglobin
or glycohemoglobin test, is used to monitor the
effectiveness of therapy in people already diagnosed
with diabetes. This test measures the amount of
glucose attached to hemoglobin (the oxygen-carrying
protein in red blood cells), which increases as
blood glucose levels rise. Since hemoglobin circulates
in the blood until the red blood cells die (half
the red blood cells are replaced every 12 to 16
weeks), the HbA1c test is a useful tool for measuring
average blood glucose values over the previous
two to three months.
Purpose of the Test
fasting, postprandial, and oral glucose tolerance
tests are used to diagnose type 1 or type 2 diabetes
is used to monitor the effectiveness of dietary
or drug therapy in the management of diabetes mellitus.
Who Performs It
lab technician or nurse.
wide variety of factorsincluding medications,
some herbs and supplements, diet, recent illness,
pregnancy, infection, stress, smoking, caffeine,
and strenuous exercisemay affect blood glucose
levels and interfere with the accuracy of the results.
Before the Test
to your doctor any medications, herbs, or supplements
you are taking. You may be advised to discontinue
some of these agents before all of these tests,
Fasting glucose test:
12 to 14 hours before the test (but no longer
than 16 hours). Water is permitted.
Postprandial glucose test:
blood glucose test is performed to establish
the pre-test blood glucose level.
You will be instructed to
eat a balanced meal containing at least 75 grams of carbohydrates,
and then fast for two hours.
Do not smoke or perform any
strenuous activities after the meal.
Oral glucose tolerance test:
will advise you to maintain a high-carbohydrate
diet for three days before the test.
You should fast for 12 to
14 hours before the test. Water is permitted.
Do not smoke, exercise strenuously,
or drink coffee or alcohol for eight hours before the test.
Your doctor may weigh you
to determine the appropriate amount of glucose to include in
the test beverage.
You may want to bring something
to read during the test.
preparation is necessary.
What You Experience
Fasting, postprandial, and HbA1c
of your blood is drawn from a vein, usually in
your arm, and sent to a laboratory for analysis.
Oral glucose tolerance test:
be drawn using venipuncture and a urine specimen
will be obtained to determine fasting glucose
You will be given a glucose-laden
beverage. It will be very sweet. (It may be diluted with a small
amount of lemon juice and water.)
You won't be allowed to eat
anything until the test is complete, but you may drink water.
Additional blood samples
are taken at 30 minutes, 60 minutes, and hourly intervals after
you drink the glucose beverage.
Urine specimens are taken
at hourly intervals.
The total test time is usually
three hours but may last up to six hours.
Risks and Complications
fasting, postprandial, and HbA1c tests are associated
with no risks.
may experience symptoms of hypoglycemia (weakness,
restlessness, hunger, sweating, nervousness) during
the oral glucose tolerance test. Tell your doctor
immediately if this happens. If these symptoms
persist, you will be given orange juice, and the
test will be discontinued.
After the Test
blood is drawn, pressure is applied (with cotton
or gauze) to the puncture site.
may be given a snack or some orange juice after
the oral glucose tolerance test.
may resume your normal diet, activities, and any
medications withheld before the test.
may collect and clot under the skin (hematoma)
at the puncture site; this is harmless and will
resolve on its own. For a large hematoma that causes
swelling and discomfort, apply ice initially; after
24 hours, use warm, moist compresses.
tests are performed on the blood and urine samples
to measure the level of glucose. These results
and the presence of risk factors for diabetes will
help your doctor in making a diagnosis.
o If you test positive for diabetes, the condition can be treated with dietary
measures, exercise, and, if necessary, oral glucose-lowering medications or
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