Tests for diagnosing diabetes and pre-diabetes
The following tests are used for diagnosis of diabetes
and pre diabetes:
A fasting plasma glucose test measures your blood glucose
after you have gone at least 8 hours without eating. This
test is used to detect diabetes or pre-diabetes.
An oral glucose tolerance test measures your blood glucose
after you have gone at least 8 hours without eating and 2
hours after you drink a glucose-containing beverage. This
test can be used to diagnose diabetes or pre-diabetes.
In a random plasma glucose test, your doctor checks your
blood glucose without regard to when you ate your last meal.
This test, along with an assessment of symptoms, is used to
diagnose diabetes but not pre-diabetes.
Positive test results should be confirmed by repeating the
fasting plasma glucose test or the oral glucose tolerance
test on a different day.
Fasting Plasma Glucose (FPG) Test
The FPG is the preferred test for diagnosing diabetes due
to convenience and is most reliable when done in the morning.
Results and their meaning are shown in table 1. If your fasting
glucose level is 100 to 125 mg/dL, you have a form of pre-diabetes
called impaired fasting glucose (IFG), meaning that you are
more likely to develop type 2 diabetes but do not have it
yet. A level of 126 mg/dL or above, confirmed by repeating
the test on another day, means that you have diabetes.
| Table 1. Fasting Plasma
Glucose Test |
| Plasma Glucose Result (mg/dL) |
Diagnosis |
| 99 and below |
Normal |
| 100 to 125 |
Pre-diabetes(impaired fasting glucose) |
| 126 and above |
Diabetes* |
Oral Glucose Tolerance Test (OGTT)
Research has shown that the OGTT is more sensitive than the
FPG test for diagnosing pre-diabetes, but it is less convenient
to administer. The OGTT requires you to fast for at least
8 hours before the test. Your plasma glucose is measured immediately
before and 2 hours after you drink a liquid containing 75
grams of glucose dissolved in water. Results and what they
mean are shown in table 2. If your blood glucose level is
between 140 and 199 mg/dL 2 hours after drinking the liquid,
you have a form of pre-diabetes called impaired glucose tolerance
or IGT, meaning that you are more likely to develop type 2
diabetes but do not have it yet. A 2-hour glucose level of
200 mg/dL or above, confirmed by repeating the test on another
day, means that you have diabetes.
| Table 2. Oral Glucose
Tolerance Test |
2-Hour
Plasma Glucose Result (mg/dL) |
Diagnosis |
| 139 and below |
Normal |
| 140 to 199 |
Pre-diabetes (impaired glucose tolerance) |
| 200 and above |
Diabetes* |
*Confirmed by repeating the test on a different day.
Gestational diabetes is also diagnosed based on plasma glucose
values measured during the OGTT. Blood glucose levels are
checked four times during the test. If your blood glucose
levels are above normal at least twice during the test, you
have gestational diabetes. Table 3 shows the above-normal
results for the OGTT for gestational diabetes.
| Table 3. Gestational
Diabetes: Above-Normal |
| Results for the Oral
Glucose Tolerance Test |
| When |
Plasma Glucose Result (mg/dL) |
| Fasting |
95 or higher |
| At 1 hour |
180 or higher |
| At 2 hours |
155 or higher |
| At 3 hours |
140 or higher |
Note: Some laboratories use other numbers
for this test.
For additional information about the diagnosis and treatment
of gestational diabetes, see the NIDDK booklet What I Need
to Know About Gestational Diabetes.
Random Plasma Glucose Test
A random blood glucose level of 200 mg/dL or more, plus
presence of the following symptoms, can mean that you have
diabetes:
Increased urination
Increased thirst
Unexplained weight loss
Other symptoms include fatigue, blurred vision, increased
hunger, and sores that do not heal. Your doctor will check
your blood glucose level on another day using the FPG or the
OGTT to confirm the diagnosis.
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