Fasting glucose (FBS) test: normal range

A fasting glucose test measures the glucose, or sugar, in your blood after at least eight hours with nothing to eat or drink except water. Glucose comes from the food you eat and is your body's main source of energy. Blood glucose normally rises a little after a meal, so testing after a fast removes that rise and shows your baseline level at a single point in time.

Doctors order fasting glucose to screen for prediabetes and diabetes, often from age 35, and to follow blood sugar in people already diagnosed. It is also used to check for side effects of medicines that change blood glucose. A single high result is not a diagnosis on its own: NIDDK says a doctor usually confirms it with a second test.

NIDDK suggests adults with normal results are retested every three years and people with prediabetes every year. Fasting glucose is usually read beside HbA1c, which shows the average over three months rather than one morning.

Fasting glucose moves with what you ate, when you ate it and what else is going on in your body.

• A fast shorter than eight hours, or anything other than water during it, means the sample is not a true fasting sample.
• Stress from surgery, a serious illness or an injury can raise blood glucose.
• Some medicines, such as steroids and some blood pressure, cholesterol and mental health medicines, can raise glucose, while others can lower it. Tell the lab and your doctor about every medicine you take.
• Being more active than usual, or eating too little, can lower glucose, mainly in people on diabetes medicines.
• Conditions of the thyroid, pancreas, liver, kidneys or hormone glands can push the result up or down, which your doctor considers.
• In some thin, young people a fasting level below 70 mg/dL can be normal.