Fasting blood glucose measures the concentration of glucose in the blood after at least 8–12 hours without food or caloric beverages. In the fasted state, blood glucose is maintained primarily through the liver's release of stored glucose (glycogenolysis) and glucose production from amino acids (gluconeogenesis), both regulated by insulin and glucagon.

Fasting glucose is the primary screening test for diabetes and pre-diabetes. A fasting glucose of 5.5–6.9 mmol/L indicates impaired fasting glucose (pre-diabetes); 7.0 mmol/L or above on two occasions confirms diabetes. Fasting glucose should always be interpreted alongside HbA1c and, where possible, fasting insulin — as insulin resistance may be present with a normal fasting glucose.