There are three tests that can diagnose diabetes:
- Fasting plasma glucose (FPG)
- Oral glucose tolerance test (OGTT)
- Hemglobin A1C (A1C)
Fasting Plasma Glucose Test
The fasting plasma glucose (FPG) test has been the standard test for diagnosing diabetes. It is a simple blood test taken after 8 hours of fasting.
FPG levels indicate:
- Normal. 100 mg/dL (or 5.5 mmol/L) or below.
- Pre-Diabetes. (A risk factor for type 2 diabetes): Between 100 - 125 mg/dL (5.5 - 7.0 mmol/L).
- Diabetes.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. Widespread screening of patients to identify those at higher risk for diabetes type 1 is not recommended.
Oral 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 doctors 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.
OGTT levels indicate:
- Normal. 140 mg/dL or below.
- Pre-Diabetes. Between 140 - 199 mg/dL.
- Diabetes. 200 mg/dL or higher.
Review Date: 05/05/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.