Diagnosis
Testing for Glucose Abnormalities
Fasting Plasma Glucose. The fasting plasma glucose (FPG) test is the standard test for diagnosing diabetes. It is a simple blood test taken after 8 hours of fasting. In general, results indicate the following:
- FPG levels are considered normal up to 100 mg/dL (or 5.5 mmol/L).
- Levels between 100 and 125 mg/dL (5.5 to 7.0 mmol/L) are referred to as impaired fasting glucose or pre-diabetes. These levels are considered to be risk factors for type 2 diabetes and its complications.
- Diabetes is diagnosed when FPG levels are 126 mg/dL (7.0 mmol/L) or higher.
The FPG test is not always reliable, so a repeat test is recommended if the initial test suggests the presence of diabetes, or if the tests are normal in people who have symptoms or risk factors for diabetes. For example, people who take the test in the afternoon and show normal results may actually have abnormal levels that would be revealed if they are tested in the morning.
Glucose Tolerance Test. The oral glucose tolerance test (OGTT) is more complex than the FPG and may overdiagnose diabetes in people who do not have it. Some experts recommend it as a follow-up after FPG, if the latter test results are normal but the patient has symptoms or risk factors of diabetes. The test uses the following procedures:
- It first uses an FPG test
- A blood test is then taken 2 hours later after drinking a special glucose solution
The following results suggest different conditions:
- OGTT levels are normal up to 140 mg/dL
- Levels between 140 mg/dL and 199 mg/dL are referred to as impaired glucose tolerance or pre-diabetes
- Diabetes is diagnosed when OGTT levels are 200 mg/dL or higher
Both the FPG and OGTT tests require that the patient not eat for at least 8 hours prior to the test.
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