Fasting blood sugar levels are self-explanatory to some extent in that they are the blood glucose results you get when undertaking a period of fasting.

Fasting is frequently deemed as being at least 8 hours after taking nutrition.

This means that blood glucose levels will be taken at a time when your body is less likely to be digesting food.

Why measure fasting blood glucose levels?

Measuring fasting blood sugar levels can be useful for a number of different reasons including:

  • Diagnosing prediabetes and type 2 diabetes
  • Monitoring the extent of glucose intolerance in people with insulin resistance
  • Setting basal insulin rates in people with type 1 diabetes

How is a fasting blood glucose test carried out?

A fasting blood glucose test can be carried out either with blood taken from your arm which is tested in a lab, or it can be carried out with a finger prick blood test and a blood glucose meter.

A fasting blood glucose involves fasting, not taking food or any non-water drink, for at least 8 hours.

A lab tested sample, also known as a fasting plasma glucose test, provides a more accurate result so this method will be used to diagnose or monitor glucose intolerance.

When testing to inform basal dose setting of insulin, results from a blood glucose meter will be sufficient.


A fasting blood glucose test can be used to diagnose diabetes or Impaired Fasting Glycemia, a condition that has a high risk of developing into type 2 diabetes.

Condition indicated Blood glucose level (mg/dl)
Normal Under 100
Impaired Fasting Glycemia 100 to 125
Diabetes 126 or more

Monitor glucose intolerance

If you have a form of glucose intolerance such as pre-diabetes or type 2 diabetes, your health team may wish to carry out a fasting plasma glucose test to monitor how well your body copes with maintaining blood sugar levels when fasting.

This can be helpful for doctors when choosing the best treatment for you.

Set basal insulin rates

Fasting blood glucose tests can also help people with type 1 diabetes to choose the right dose to set their background insulin at. Background insulin, also known as basal insulin, is long acting insulin which is designed to counteract the body’s natural release of glucose during the day.

Fasting blood glucose tests may be particularly useful in the following situations:

  • If starting on a new type of background insulin or insulin regimen
  • When going through pregnancy with type 1 diabetes
  • When starting or finishing adolescence
  • If your basal insulin needs reviewing for any other reason

Your diabetes team can guide you on whether your background insulin needs to be reviewed and how best to go about testing your blood glucose levels.