The FPG test is the preferred test for diagnosing diabetes because of its convenience and low cost. However, it will miss some diabetes or pre-diabetes that can be found with the OGTT. The FPG test is most reliable when done in the morning. Results and their meaning are shown in Table 1. People with a fasting glucose level of 100 to 125 milligrams per deciliter (mg/dL) have a form of pre-diabetes called impaired fasting glucose (IFG). Having IFG means a person has an increased risk of developing type 2 diabetes but does not have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.
Table 1. FPG test
|Plasma Glucose Result (mg/dL)
|99 or below
|100 to 125
(impaired fasting glucose)
|126 or above
|Confirmed by repeating the test on a different day.
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 fasting for at least 8 hours before the test. The plasma glucose level is measured immediately before and 2 hours after a person drinks a liquid containing 75 grams of glucose dissolved in water. Results and their meaning are shown in Table 2. If the blood glucose level is between 140 and 199 mg/dL 2 hours after drinking the liquid, the person has a form of pre-diabetes called impaired glucose tolerance (IGT). Having IGT, like having IFG, means a person has an increased risk of developing type 2 diabetes but does not have it yet. A 2-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.
Table 2. OGT Test
|2-Hour Plasma Glucose Result (mg/dL)
|139 and below
|140 to 199
(impaired glucose tolerance)
|200 and above
|*Confirmed by repeating the test on a different day.
Gestational Diabetes Diagnosis:
Gestational diabetes is also diagnosed based on plasma glucose values measured during the OGTT, preferably by using 100 grams of glucose in liquid for the test. Blood glucose levels are checked four times during the test. If blood glucose levels are above normal at least twice during the test, the woman has gestational diabetes. Table 3 shows the above-normal results for the OGTT for gestational diabetes.Note: Some laboratories may use other numbers for this test.
Table 3. Gestational diabetes: Above-normal results for the OGTT*
||Plasma Glucose Result (mg/dL)
||95 or higher
|At 1 hour
||180 or higher
|At 2 hours
||155 or higher
|At 3 hours
||140 or higher
|*These numbers are for a test using a drink with 100 grams of glucose.
Random Plasma Glucose Test:
A random, or casual, blood glucose level of 200 mg/dL or higher, plus the presence of the following symptoms, can mean a person has diabetes:
- increased urination
- increased thirst
- unexplained weight loss
Other symptoms can include fatigue, blurred vision, increased hunger, and sores that do not heal. The doctor will check the person’s blood glucose level on another day using the FPG test or the OGTT to confirm the diagnosis.
The A1C Test:
The A1C test gives a snapshot of your blood glucose level over the past two to three months. An A1C level of 6.5% or more is consistent with the diagnosis of diabetes. A fasting plasma glucose test is another option. You must not eat for eight hours before the test. A result above 126 is considered diabetes. An oral glucose challenge test with a two-hour blood test may also help your doctor make a diagnosis.
C-peptide blood levels can indicate whether or not a person is producing insulin and roughly how much. Insulin is initially synthesized in the form of proinsulin. In this form the alpha and beta chains of active insulin are linked by a third polypeptide chain called the connecting peptide, or c-peptide, for short. Because both insulin and c-peptide molecules are secreted, for every molecule of insulin in the blood, there is one of c-peptide. Therefore, levels of c-peptide in the blood can be measured and used as an indicator of insulin production in those cases where exogenous insulin (from injection) is present and mixed with endogenous insulin (that produced by the body) a situation that would make meaningless a measurement of insulin itself. The c-peptide test can also be used to help assess if high blood glucose is due to reduced insulin production or to reduced glucose intake by the cells.
There is little or no c-peptide in blood of type 1 diabetics, and c-peptide levels in type 2 diabetics can be reduced or normal. The concentrations of c-peptide in non-diabetics are on the order of 0.5-3.0 ng/ml.