Fasting blood glucose: A test to determine how much glucose (sugar) is in a blood sample after an overnight fast. The fasting blood glucose test is commonly used to detect diabetes mellitus. A blood sample is taken in a lab, physician's office, or hospital. The test is done in the morning, before the person has eaten. The normal range for blood glucose is 70 to 100 mg/dl. Levels between 100 and 126 mg/dl are referred to as impaired fasting glucose or pre-diabetes. Diabetes is typically diagnosed when fasting blood glucose levels are 126 mg/dl or higher.