A fasting blood glucose test — also called a fasting plasma glucose, or FPG test — measures blood glucose levels after you've gone without food for at least eight hours. It's reliable, and the results aren't affected by your age or the amount of physical activity you do. Many doctors prefer the fasting plasma glucose test because it's easy, fast and inexpensive.
To prepare, you must not eat for at least eight hours before the test. The next morning, a healthcare provider takes a single sample of your blood and sends it to a lab for analysis. Fasting blood glucose tests done in the morning, rather than the afternoon, appear to be more accurate in diagnosing diabetes. So be sure to schedule your test for first thing in the morning.
Results Your doctor will compare your results against the normal levels for fasting glucose. Normal blood glucose levels are less than 100 mg/dL. (Read as "100 milligrams of glucose for each deciliter of blood." A deciliter is 1/10th of a liter.)
- If your blood glucose measures 126 mg/dL or higher, your doctor will order a second test. If a second test done on a different day also measures 126 mg/dL or higher, you have diabetes, and you don't need any further tests to diagnose it.
- If your blood glucose measures 100 mg/dL to 125 mg/dL, your doctor will recommend another blood test. You have higher-than-normal blood glucose levels, but the levels are not high enough to be diagnosed as having diabetes. Your doctor will confirm all abnormal tests with a second test before diagnosing diabetes.
Written by award-winning health writer Bobbie Hasselbring
Reviewed by Beth Seltzer, MD
Last updated June 2008