A blood glucose test measures the amount of sugar called glucose in a sample of your blood.
Glucose is a major source of energy for most cells of the body, including brain cells. Fasting blood sugar measures blood glucose after you have not eaten for at least 8 hours. It is often the first test done to check for pre-diabetes and diabetes.
The ideal range for fasting glucose is 70.0mg/dL to 99.9mg/dL.
Source: American Diabetes Association.