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 test is normal in people who have symptoms or risk factors for diabetes.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) is more complex than the FPG and may over-diagnose 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:
- The patient first has an FPG test.
- The patient has a blood test 2 hours later, after drinking a special glucose solution.
OGGT levels indicate:
- Normal. 140 mg/dL or below
- Pre-Diabetes. Between 140 - 199 mg/dL
- Diabetes. 200 mg/dL or higher
The patient cannot eat for at least 8 hours prior to the FPG and OGTT tests.
Hemoglobin A1C Test
This test examines blood levels of glycosylated hemoglobin, also known as hemoglobin A1C (HbA1c, A1c). The results are given in percentages and indicate a person’s average blood glucose levels over the past 2 - 3 months. (The FPG and OGGT show a person’s glucose level for only the time of the test.) The A1C test is not affected by recent food intake so patients do not need to fast to prepare for the blood test.
In 2010, the American Diabetes Association recommended that the test be used as another option for diagnosing diabetes and identifying pre-diabetes.
A1C levels indicate:
- Normal. Less than 5.7%
- Pre-Diabetes. Between 5.7 - 6.4%
- Diabetes. 6.5% or higher
Review Date: 04/01/2010
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.